People tend to always paint the aggressor to be the bad guy, especially if the aggressor loses. He attacked and stole from Jayce, but couldn't actually defeat Jayce. Now, trying to take the Arcane Crystal away from an enemy state could hardly be considered villainy, but he still did steal it, and he then lost.
I think part of it is Jayce really wants people to consider Viktor a bad guy, and Jayce is handsome, charismatic, and intelligent, so he has successfully convinced a lot of people, both fans and in-lore, that Viktor is a monster. The reality is Viktor and Zac are the only ones actually trying to help the average person in Zaun (Ekko just protects his friends and family). Viktor was dealt a shitty hand, and is just making the best of it.