[AmigaBlizzard]

https://sampwiki.blast.hk/wiki/SetPlayerScore
The return value of SetPlayerScore is 1 for success and 0 for failure.

So you're setting "level" to 0 or 1 based on success or failure of using SetPlayerScore, but I don't think that's what you want.

Code:

level = SetPlayerScore(killerid,(GetPlayerScore(killerid))+1); // Should return 1 if killerid is an online player
PlayerInfo[killerid][pLevel] = level;