Alval Uvani, a certain dark elf, is a target of the dark brotherhood in my future quests; however i can kill him now even without the dark brotherhood quest o.o
What implications would killing him now have on my DB quest?

I don't think you'd be able to start the quest later if you killed him now. I've seen something before saying "one of the characters needed to complete this quest has died", meaning I couldn't finish a quest. So I'd be careful about it if I were you.

He's one of few quest characters that can be killed before the related quest.
Killing him now will cause the quest to be completed as soon as you get it.
He often gets killed by random wildlife while travelling between cities.

I agree with kamikaze here. You'll have no problem. :foodndrink: