Generally, multiplayer games exist so that players can co-operate. Whether it's shooting opponents or just working as a team to get the job done, it's been made clear by the popularity of multiplayer games over the years that we love working as a together.
But this doesn't mean that all multiplayers are made for wholesome cooperation. In fact, there's a metric ton of games that thrive on making you turn on your most trusted buddies - despite how absolutely diabolical this is.
It's especially diabolical when you don't see it coming though. In games that have a competitive element or are based around dishonesty you know what you're in for, but plenty of titles pull the wool over your eyes until the last second before revealing you need to murder a pal.
Sure, you might discover that one of your nearest and dearest is a remorseless savage capable of keeping a completely straight face while lying to you - but that's something that's probably better to find out via video games than real life circumstances.
In short, when the Mario Kart gets tense, trust no one; not even yourself.