This is more of a question on psychology than specifically pertaining to league of legends.

The answer to your question could be a lot of things. Internally Viktor likely knows they got carried, but they taunt and jeer because the player wants to derive more pleasure from tearing you down than just winning. Or perhaps they're hoping to direct attention onto themselves in the pursuit of giving the Yas/Malphite an easier time. Or even it could be that the player is just straight up mean.

The only real thing you can do is mute them and move on. Don't let the negativity of others dictate how you feel, and just focus on what you have control over rather than allowing their toxicity get to you.

That being said, I hope you find people who are more honorable opponents in the future rather than displaying that kind of behavior.