Katherine and Damon are the same level of evil, arguably, yes. However, Damon's actions and evilness are being blamed on Katherine. The characters point to Katherine as the reason he's flown off the hinge, and Damon expects Elena to take that into consideration when making his case. Katherine, at least, is not going around blaming her evilness on others, and the text isn't blaming others for it. It's the consistent handwaving of Damon's evilness that makes it worse.

We *know* that Katherine will, at some point, have to pay for her evilness by dying or something worse while Damon is there to stay. Katherine is evil, the characters understand that she causes them issues, and they're doing something about that problem. I honestly don't get why Damon has been allowed to live thus far, if we're viewing them at the same level on the scale of evilness. If anything, Damon has more consistently hurt people close to Elena and Stefan (and on purpose, with the intention of hurting/punishing them, when Katherine's motives seem to still be a bit more complex than that, and her evil is more of the Machiavellian kind where she does evil things for her own ends, and not impulsively with the intention of hurting people like Damon) than Katherine has thus far.

But Katherine is clearly our villain and Damon is our anti-hero (who will be redeemed), so while it can be argued that they're both equally evil, the text is not showing that at all. Which, on a metanarrative level, makes Damon much, much worse.