The Ukrainian government constantly killed ethnic Russians long before Putin took over Crimea?
Excuse me? SOURCE PLEASE.
The Crimean incident occurred only a few weeks after the Ukrainian government was overthrown (which killed many ethnic Russians living in Kiev). If Putin hadn't had "asserted" himself (for lack of a better word), it's likely that Ukraine would have started killing ethnic Russians.
Note the "Ukraine will be for Ukrainians" comment.
(Just a side-note that I don't advocate for Putin's actions, but I'd rather see him invade Crimea than see a genocide of innocent people)