I think that's the only case where there was no 'justification' for his actions.

Killing Jeremy - Elena hurt his feelings very much, so did Katherine. He was heartbroken, although he did something unforgivable, I can understand why he did this.

Forcing his blood on Elena - not really a crime, even though Stefan would consider this as a great crime of taking her choices away. He did it to save her life.

Leaving Stefan - not really. Stefan killed many people after that but I think Damon can't be blamed for Stefan's crimes. IT's his fault because for all his life he preferred to play saint martyr instead of learing how to control his vampire nature.

Killing Lexi was for me the worst thing, because I can't see any mitigating circumstances here. He wasn't heartbroken, she wasn't a real danger to him. He did it, because it was profitable for him at that moment.
And that's why I can't forgive him killing Lexi.

And also forcing his blood on Elena

by far this.

Everything NocKairu said =)

and killing Jeremy was terrible too. but i really liked Lexi

Killing Jeremy, and killing Lexi. They were both terrible. The other ones were not a big deal I don't think. Him forcing his blood into Elena was not cool, but at the same time it was for her sake.

This was just mean. Ugh :@

She had so much potential as a character, and killing her on his brother's birthday? That is just horrible.

Poor Lexi

He knew how Elena loves his brother and he didn't care

I think in itself killing Lexi and Jeremy were both equally bad, but he killed Jeremy because he lost emotional control, while he killed Lexi out of cold calculation.

and Lexi