Jesus did not go around saying, "Hey let's be nice," nor did he get crucified for saying that. Jesus called out the Jewish leaders of the day for being hypocrites, people who laid a heavy burden on their followers but did not live up to the standards they set, who made up and imposed strict rules while completely missing the point. They crucified him because they felt threatened by him, that their power structure would collapse, and then they wouldn't be powerful anymore. Nothing's changed in that aspect of humanity--the powerful always want to preserve their power.

He did not call people to be nice, he called people to be good and faithful. There is often a big difference between being nice and being good. By contemporary standards, if Jesus had gone around being "nice," he never would have confronted anyone or accused them of sinning.

Jesus certainly did get angry sometimes, but his anger was justified, and he did not lose self-control.

You can't separate Christianity or Christ from God. Think about this: if Jesus was not who he claimed to be--the son of the living God--then he was surely crazy, because he got himself killed for nothing. He could have avoided all that suffering and death by just being nice and non-confrontational, and going to a place where people didn't want to kill him.