I'll try to keep both perspectives in mind with this comment. I don't mean to give a cop out answer, but it honestly varies from person to person. For someone like yourself who plays for enjoyment, you'd probably rather have the gold that you worked to earn yourself. For someone like the Volibear who I'm assuming plays more so to win, he's going to opt for whatever tactics will win the game. If that means taking a few kills from your teammates, then so be it. I don't even have to lolking you to tell that this was Ranked solo queue, where even more emphasis is placed on winning since there's something at stake (as opposed to normal games).
I won't condone his negative behavior (the gloating, etc), but I can understand his reasoning for picking up the kills. Trusting others, especially in solo queue, can be risky. You know what you're capable of in your elo, but you don't have any real idea as to your teammates. What if he gave you the kills and you did nothing with them but afk farm? What if you continually got caught out of position and therefore couldn't contribute to the team? Suddenly he's in a bad position where he has to rely on you more so than one would care to when he could have taken the early kills and contributed himself. I'm not saying that the above would have happened, but these are common thoughts of those going through solo queue players' heads. We've all had that Vayne player that got retardedly fed, then threw the game trying to 1v5 the enemy team.
For me, it comes down to whether or not he made efficient use of the gold, which he did. You won the game, and you'll probably never see him again in your life. Next match. But if it were a normal, then I'd probably lean more towards your current perspective: Sure, we're winning, but this isn't very enjoyable for me.
TL;DR If you're playing to win, winning should be your #1 objective. How you win is irrelevant. If they're being toxic or rude, mute them and report end game.
