If Buddha had unconditional love, why did he "kill" some of his monks?

When you love your childen, do you love them when they are Good but not when they are Bad? No, you love them always, despite their shortcomings, but you still punish them for their own sake, exactly because you love them.

I edited my answer, it is being peer-reviewed. Lost access to that account, so recreated it.

I am an elder brother(through body) of someone whom I care-for, and a good monk too. I need to maintain my behaviour in such a way that, "both my brother and those aspiring to become like me can follow me, can make me as their role-model, can have faith on me... " Similar to this. It doesn't mean that I am too attached to them or I want to neglect them, it's just the way as how they would understand me.

Being a very serious-Freak meditator, I also know that sometimes when I have to teach some of the images within my brain, and if the real-ones(of the corresponding images) don't obey me, then I have to stop em-powering thoughts/intentions regarding teaching those wrong-audience/s... a type of killing.

It doesn't mean that, "I have no metta for them or I ignore them", rather being a freakish-meditator, I give them my compassion to such extent that, on later stage of life when they come to right-senses, they don't remorse for their ignorance.

It also doesn't mean that, "I am OK for their moral, spiritual declination or increment in their cravings for wealth", if such would have been the case, I would've never taught them as I have enough funding already.

Compassion: An act of response for all of them whom you want to be in peace. You may need to use some harsh words or neglecting-type behaviour.

The "killing" reference would be this one probably -- Kesi Sutta (AN 4.111)

Perhaps that's analogous to a doctor's not giving medicine to a patient who won't benefit from it.

And perhaps the Buddha had the welfare of others -- the whole Sangha -- to consider too.

Another instance is Channa in DN 16.

And there are a few other examples on pages 89-90 of Piya Tan's analysis of the Kesi Sutta:

The untamable

Those who cannot be tamed. While most of the suttas record how the Buddha’s teachings convert many and inspire others, there is a handful of important discourses where, apparently, the protagonists lack the capacity for spiritual change. Interestingly, in all such cases, the Buddha does indeed teach them the Dharma. However, for some reason, the protagonists remain unmoved despite being admonished by the Buddha himself.

Such discourses are very few, that is, where the protagonists (one may even call them “antagonists”) are “dead” to the Dharma-Vinaya. The best known of these are:

• the Mahā Taṇhā,saṅkhaya Sutta (M 38), where the monk Sāti holds on to the wrong view that our consciousness passes on unchanged in rebirth;34
• the Alagaddûpama Sutta (M 22), where the monk Ariṭṭha wrongly views that sensual pleasures (including sex) are not “stumbling blocks” to the celibate holy life;35
• the Udumbarikā Sīha,nāda Sutta (D 25), where, despite listening and accepting a long inspiring discourse from the Buddha, the wanderer Nigrodha and his followers still neither renounce nor go for refuge.

The most notorious case of one who is “dead” to the Dharma-Vinaya is clearly the monk Deva,datta. Despite being the Buddha’s cousin and listening to numerous Dharma teachings, he still shows antagonism against the Buddha, desiring to take over the leadership of the monastic sangha from the aging Buddha. The Devadatta narrative where he is depicted as the Buddha’s antagonist is, however, not without problems, and should be carefully studied to understand its significance.

The most famous case of a person who seems to be intractable is the elder Channa, the erstwhile charioteer of the Bodhisattva himself.38 On account of Channa’s closeness to the Bodhisattva and then the Buddha, he became arrogant, thinking “Our Buddha, our Dharma,” besides his inherited conceit of being a kshatriya (khattiya,māna). Channa’s “butler” narcissism and arrogance were, in the end, selfdefeating. He failed in his tasks as a monk even while the Buddha lived. While dwelling in Kosambī, he was unwilling to acknowledge an offence he committed that the Buddha decreed a formal act of suspension (ukkhepaniya,kamma), forbidding him commensality (eating together) and dwelling with the monks.

He continued to commit other offences, especially disrespecting and reviling senior monks. One of the Buddha’s last acts was to instruct Ānanda to impose the “supreme penalty” (brahma,daṇḍa), that is, a total boycott of Channa by the sangha.41 After the Buddha’s death, Ānanda executed the order.

The closing of Culla,vagga 11 of the Vinaya records how when Ānanda conveys the supreme penalty to Channa at Ghosit’ārāma, in Kosamb, he faints at the thought of being boycotted by the order. Almost immediately Channa diligently works to correct himself—as movingly reported in the (Lakkhaṇa) Channa Sutta (S 22.90; V 2:292).

The (Lakkhaṇa) Channa Sutta (S 22.90) records in an almost humorous tone, how he attempts to seek Dharma instruction from other monks. Apparently, no monk actually boycotts him, since they try to answer his questions, albeit careful to not offend him. Finally, Ānanda teaches him the Kaccāna,gotta Sutta (S 12.15) on how dependent arising counters the two extreme views of eternalism and annihilationism, and how self-view is replaced by the realization that it is only dukkha that arises and ceases—and he breaks through to the Dharma. The supreme penalty is automatically lifted at the moment of Channa’s final attainment.

This is perhaps the only account we have of any monastic close to the Buddha who is apparently declared “dead” to the sangha. However, by his own efforts, Channa returns alive and well to be with the sangha, in due course, as an arhat.

The Kesi sutta is about tameable and untameable. I think you've quoted "little dust in their eyes" yourself. Maybe it's not appropriate to question how the Buddha handles the supposedly "untameable".

Whether the Buddha behaves morally and maintains a mind of good-will is presumably unconditional, unaffected by the behaviour or temporary misbehaviour of others.

You might read Positive Response -- How to Meet Evil With Good for example if you can.

It suggests that "tit for tat" is a common strategy in the world but, it says, never works in the realm of spirituality. It references the (famous) Parable of the Saw, for example.

One other thing I'd note about the Buddha is his ability to enter the minds of others and to know when they are ready or not ready -- I think you see that in suttas like AN 8.30.

It might be a mistake for you to imagine that you can do the same -- to enter the minds of others.

It might also be a mistake to assume that they can behave unconditionally -- and unaffected by real or imaginary misbehaviour of yours, by their own projections, etc.

But by definition it means to try to behave well and with good will even if someone else doesn't.