If Christians believe that Christ was a God and immortal (or resurrected), why do some avenge his killing in his name?

I don't think I've ever heard the argument phrased in quite this way, but Antisemitism is un-Christian. As it happens, I was thinking of this question while studying one of the standard Advent texts:

For to us a child is born,
    to us a son is given;
and the government shall be upon his shoulder,
    and his name shall be called
Wonderful Counselor, Mighty God,
    Everlasting Father, Prince of Peace.
Of the increase of his government and of peace
    there will be no end,
on the throne of David and over his kingdom,
    to establish it and to uphold it
with justice and with righteousness
    from this time forth and forevermore.
The zeal of the LORD of hosts will do this.

—Isaiah 9:6-7 (ESV)

The first verse should be a familiar text to most Christians. We believe that Jesus was and is that child. The title is, perhaps, the best expression of what Christians believe about Jesus that isn't written in the New Testament. It's also an important piece in the Trinitarian puzzle. The messianic application of the passage to Jesus won't likely be abandoned by many Christians.

The second verse is equally powerful and theologically useful, but it includes some phrases that should be troubling to those who espouse the view mentioned in the question:

• "the throne of David"

  The kingdom Jesus will establish is Davidic and that means Jewish. In both the New and the Old Testaments, it's clear that the King appointed by God will be a descendant of Abraham via Judah and David. Both Luke and Matthew show that Jesus fit that lineage.

• "with justice and with righteousness"

  If justice is to be served by punishing the decedents of the people who crucified Jesus, then it will be the Davidic king who will establish righteousness. But I don't believe that justice would be served. Jesus did not condemn children for their parent's sin:

  And his disciples asked him, “Rabbi, who sinned, this man or his parents, that he was born blind?” Jesus answered, “It was not that this man sinned, or his parents, but that the works of God might be displayed in him. We must work the works of him who sent me while it is day; night is coming, when no one can work. As long as I am in the world, I am the light of the world.”—John 9:2-5 (ESV)

• "The zeal of the LORD of hosts will do this."

  Again, the Kingdom of God is advanced by God and not merely by the will of people. Christians believe (and rightly so) that we are the vanguard of God's Kingdom. But Jesus never commanded us to kill His enemies. Rather, we are able to help Him break into the world with kindness:

  Bless those who persecute you; bless and do not curse them. Rejoice with those who rejoice, weep with those who weep. Live in harmony with one another. Do not be haughty, but associate with the lowly. Never be wise in your own sight. Repay no one evil for evil, but give thought to do what is honorable in the sight of all. If possible, so far as it depends on you, live peaceably with all. Beloved, never avenge yourselves, but leave it to the wrath of God, for it is written, “Vengeance is mine, I will repay, says the Lord.” To the contrary, “if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals on his head.” Do not be overcome by evil, but overcome evil with good.—Romans 12:14-21 (ESV)

My opinion on the matter is based on 2 assumptions:

• We are humans and we have a tendency to misunderstand events.
• Knowledge of Jesus' death is very widespread.

With these two assumptions in mind, my opinion is almost guessable...

As a Christian I believe that Jesus died for my sins, because of my sins; if we were sinless, Jesus would have died for nothing. So, to summarize, I (and other Christians I know) believe that I killed Jesus, by being a sinner -- the reason he died. So, in my mind, the Jews are just as responsible for Jesus' death as I am.

Now, because so many people know of Jesus' death and people have a tendency to misunderstand things they see the Jews as being the murderers of Christ without looking deeper. They see the action, but they ignore the cause; they are judging the effect but not the affecter. This is an unfortunate misinterpretation, and it is sad that anyone would be inclined to seek "vengeance" for such things. This is, of course, just my opinion.

It is a stretch to call the "retaliation" you mention a response to Jesus' murder. It is more accurately described as a response to differing belief systems, oppression of belief, etc. It is also widely accepted that early Christians were at least themselves convinced that Jesus physically resurrected, so it would take a lot more than psychology based speculation to overturn our current understanding.

Furthermore, the analogy you offered is a bit misleading. You talk about a father's "death" but Jesus' "murder." If someone watched their father get murdered, and then resurrected, I would imagine they would at first feel joy and awe (as the early Christians did), and later, especially after the father was gone, begin to feel anger and possibly vengeful (depending on who it was).

I think the major flaw in your reasoning is that it seems to assume that the desirability of the outcome of an event erases the bad intentions of those who brought it about. That's simply not true.

Nnowhere in scripture, or any Christian theology that I know of, are Christians commanded to avenge Christ's killing. In fact there is good evidence to say that God opposes it: "'Vengence is mine', sayeth the Lord". So any Christians who are taking revenge on any group because they believe them to be responsible for Jesus' death are not acting according to Christian principles. I'm not of course saying it hasn't happened - but since it's not in accordance with Christian principles then you aren't going to find any logical or theological justification.