The phrase, "nice guys finish last," can often be looked at from several different angles. It fits into video games pretty perfectly, and it's definitely no different in Overwatch.
We've all been in a situation where you're emoting with the enemy, having fun, and then suddenly one of their teammates comes over and doesn't hesitate on killing you. When the bond is broken, all rules are off the table.
In this clip by Reddit user urthsin, it shows exactly why sometimes it's best to turn off all chat when playing your favorite game.
I think we've all seen stuff like this happen before, but every time, I can't help but feel sorry for the victim. It's a mixture of hilarity and sadness that just can't be matched by anything else.
I guess, "keeping your enemies closer," might get thrown off the table as well in scenarios like this one -- especially if your enemy is playing Widowmaker.