Personally, I always find it worrying and slanted when I see people banging on about lefties, and left-wing, usually in derogatory ways - it serves not to open honest discussion and debate, but to insult and shut-down debate.
I also find that while it may be valid to point out genuine documented and proven left-wing terrorism, most of the attempts I've seen are anything but proven and most seem more intent on insulting and mud-slinging, trying to make out their side is some how better. So, in short... where I've seen this kind of thing, it's usually accusing left-wing of this and that, while overlooking and trying to deflect from their own side's bad-behaviour.