And since they are considered evil...
By whom? By your humanesque sense of morality? There's a tautology hidden here, my friend.
By whom you ask?
I'd say by pretty much the entire Human and Vasudan races in FS2 and a lot of real people too.
Exactly. That is why I said it was tautological. It's still true, shivans are evil, despite this fact being tautological: we consider everything we "don't like" as evil, and if we don't like what shivans are doing, they are evil. Seems simplistic in logical terms, but it works.
Me? I think what they do is wrong. That doesn mean they are necessarily evil, though wiping out entire planets, sunsystems and races is hard to justify. Especially since they didn't even try to communicate (or at least didn't put enough effort into it that Humans and Vasudans were able to even notice it).
Notice, all this is human values, not shivan nor "alien". What is "wrong" in wiping out entire species, apart from humans not liking it? How many times you kill annoying ants crawling up the kitchen table, without a second's thought? Do you consider yourself "evil" for doing so?
And this obsession with communication might well be just a human folly. Perhaps it's precisely this obsession that the shivans are keen to wipe the galaxy from, as some form of pollution. Ever thought about that?