Well, remember that League is a team game, and getting kills isn't everything. Relax, take it easy, and remember that it is a game to enjoy and have fun.
KS = kill secured where, where having someone else jump in to get the kill is still better for the team since the team gets 1.5 times what you would have gotten if you got the solo kill from that extra assist (you will still get half of the value of the kill). And it's absolutely essential for someone like Xin to get kills early game, where his entire centers around him jumping into the enemy team where he will either be useful where he can jump in, deal a lot of damage and survive, but if he is behind, he just won't be able to do much. If he doesn't get kills in the early game, he will fall behind very quickly since he can't clear the jungle as well as some of the other junglers.
You are still getting the assist gold, and you definitely want Xin to get fed...Would you rather have a fed Xin that jumps into the enemy team and have the enemy team panic and try to deal with him while the rest of your team deals damage, or would you prefer a Xin that doesn't deal much damage that will get bursted down before he could do anything and have the fight instantly becoming 5 vs 4, or to have Xin not go in and have the enemy team try to burst you down?
And Xin jumping into the enemy team... well, it's kind of like Lee Syndrome (when Lee Sin follows up on his Sonic Wave no matter what) where Xin will jump into the enemy team a lot... definitely a bad idea jumping into a team with Fiddlesticks and Thresh.