(Question: If a Christian blows up a building, does that make them a Christian Extremist?)

No, it means he was not a Christian to begin with. According to the Bible no one who hates or murders is a Christian.

The person was probably an atheist pretending to be a Christian. A wolf in sheep's clothing.

1Jn 4:20 If a man say, I love God, and hateth his brother, he is a liar: for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen?

1Jn 3:15 Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him.