For good people to do bad things...

[Ryft]

If the appalling acts in the Old Testament are Christian the perform ...

That made my brain explode. I can't even guess at what you were trying to say to edit it.

Am I wrong in reading that Jesus supported the Old Testament?

Maybe. It depends entirely on what you mean by "supported." Jesus was the promised Messiah that the old covenant system pointed toward. He supported the Old Testament in the sense of explaining and fulfilling its messianic promises, including the arrival of a new covenant system between God and his chosen people in Christ. The sense in which Jesus "supported the Old Testament" is carefully laid out in the books of John, Romans, Galatians, etc., especially the book of Hebrews.

What I mean is that Christians interpret the Bible in different ways. And while it may be considered unchristian to do such appalling acts, how could you say Christians aren't Christians for doing them?

Again, as I already explained, "murderous violence and terrorist activity follows from their beliefs, but those beliefs are not Christian beliefs. Christianity neither commands nor condones such appalling acts; more than this, it contradicts them quite diametrically" (Christianity being that doctrinal body constituting the new covenant under Jesus Christ). If you hold that the violent destruction of abortion providers can be substantiated by cherry-picked verses from Christian scriptures, please provide one or more examples of such. Otherwise your assertion is devoid of any reality. I believe it was Christopher Hitchens who advanced the maxim, "What can be asserted without proof can be dismissed without proof." (Note: If you cannot find even the hint of a "cherry-pick" example from the New Testament, that ought to tell you something about the state of it being Christian.)