Why did the Buddha suffer before he died?

Bear in mind that there's Suffering and suffering. "suffering" (lower-case "s") is the typical way in which the word is used. It is the workaday definition of "suffering", i.e., to be in discomfort, to not like something because it brings about physical or mental anguish: being hungry, in pain, heartbroken, in grief, etc. Believe it or not, these kinds of sufferings are rooted largely in our physiology, i.e., our nervous systems. We are wired to be repelled by pain, want to sate hunger, have mate(s), etc., because it is how we have evolved as beings, and for good reason. How long would any living thing live if it could simply ignore hunger or pain, or worse still, actually like it, esp. hunger? Such a species would quickly die out, and that is contrary to life itself. That we are alive shows our nature as living beings in this regard: we are alive because we do that which allows us to live, and that includes finding ways to sate hunger, address our wounds, find mate(s), etc. Living things that don't do that quickly die out.

"Suffering" (capital "S") is a different sort; it's "big-picture" kind of Suffering. All living things suffer (small "s"), and Buddha taught that and lived it (well, he had no choice). "Suffering" however is a choice. When a being chooses to accept as stimulation (or input) any sensation, feeling, or thought and simply acknowledge it without attaching to it (i.e., without identifying with it as being part of itself instead of it being a mere interpretation of stimuli), it is then that Suffering is avoided. There may still be suffering and that suffering is often a good thing to listen to: hungry? Get some food. Lonely? Get some company. In pain? Get some help. Lower-case "s" suffering thus is a very useful thing to have around. The mistake people make is that they over-identify with it sometimes and mistake it for being a permanent or definitive part of themselves, when in fact, it's transient input (whether generated by external force, as with pain from a medical condition, or internally, as when someone seems overly focused on pessimistic ideas). That is what Buddha taught: rising and falling, coming and going, it's all impermanent. Life itself is included in it; you were not alive before this life (well, leaving aside reincarnation ideas, at least not alive as you are today), then you are alive, then you'll die one day. So if you had never lived, it'd be all the same. You had to be born some time, so why kvetch about the fact that life is limited in duration? And if life has so much suffering in it, even for relatively well-off people such as those living middle-class or better lives in a first world country, why then would anyone really want to live forever? But our instincts naturally incline us to desire life, since that is a valuable (and evolutionarily advantageous) instinct to have. It impels us to defend ourselves against attack, get food, seek shelter, and weather all manner of hardships. So this desire to live is hardly useless. Yes, even Buddha had it. He just knew what place it held in the scheme of things and so didn't let it rule the very life that desire existed to protect.

When once asked what he was: a god, a diva, or something else, he replied no to all of them. He was then asked, well then, what are you? His reply was simple: "Awake". Well, for what that's worth, he was known for leaving people with answers that raised more questions than they answered. But he never claimed (to the best of my knowledge) to be THE most supremely enlightened being to ever walk the face of the Earth, though a lot of other people have said that about him. Hard to say if they are right but if Buddha himself never claimed it, then I wonder what makes others think that about him? I'll say this about him: he got the whole all-is-impermanent thing down pretty good. And teaching loads of people how to meditate was a pretty nice thing to have done, too. So all in all, he was a pretty decent chap. Perfect? No. No one is. And perfect by whose standards, anyway? I think he'd be the first to tell you that perfection is relative, and it is.

The Buddha was still a human being in the sense that he had a physical body as a results of the 5 aggregates.

He was once asked if he was in pain regarding his pains in the back. His answer was that there was pain but he was not bothered by it.

As an enlightened being he had destroyed all defilements and fetters and would therefore not identify or take ownership of the pain. There would be noone to do that. There would just be pain - an impersonal phenomena.

Buddha did not experience suffering(mental). He experienced pain due to poor health during his final days. Enlightened beings do not create new Karmas. Also, they have removed the potential of all their past Karmas to give future births. But they can still face the consequences while they live.