Here's a thought.
At this moment in time, people of Left persuasions are more likely to bear animosities towards Zionism and Israel than others.
It can also be said, as a general, over-arching and thus not precise statement, that Left-persuasion people are more likely to think the world can be perfected, while Right-persuasion people are dubious.
Zionism is inherently messy. It has set itself the totally legitimate and defensible goal of creating - and now, maintaining - a Jewish state; yet this inevitably has an impact on the local Arabs; the best one can hope for is a rather messy compromise. Starry-eyed Utopians don't like messiness, so they either detest the Zionists for the past crime of having messed up the area, or for being messy with the way they do their thing, or for ensuring by their very existence that the future reality will continue to be messy.
Of course, this tells us nothing about why some Left-persuasion people single out Israel for their ire. The Chinese also make the world rather messy, after all.
An over simplification, I know, but worth mentioning.