The #1 worst thing you can do when ahead

Trade kills. It will end up helping the enemy far more than it will yourself. The best way to get back into a game is get a few kills under your belt. So trading is always a good thing when behind. Especially if you are not worth too much gold or they have a bounty. When ahead, please listen, it may seem like a good idea to dive 1v5 as a fed kat and maybe get that nice penta. but if you miscalculate, even by a sliver, you die instantly and give them gold. Chasing kills that are not free is always a bad idea. Unless they have no chance of escape or rescue, or you need to buy time for something/someone else you should never chase the kill past 3-5 seconds of chasing. They will most likely have a few teammates coming to help and then the chase becomes a bait by the enemy.

Just forcing a back is plenty for early game if your farming is good and/or they are bad at farming. The gold and experience they will miss is well worth it. But trading kills will net you virtually nothing unless it is first blood. The enemy wants to trade kills when behind and will look for any chance to do so.