A bad person is going to ruin your game no matter what system you play. That’s also true for a board game. That’s also true for any social situation. Interacting with bad people makes for a bad time, no matter the context. It’s such a basic and fundamental truth that is doesn’t even bear mentioning.
You are playing a game with people, and we assume that all of those people want to play the game and get enjoyment out of it. Once you have reached this point, the game you play matters A LOT. It’s probably the most important thing. The same group of people will have drastically different experiences if they choose to play D&D, Burning Wheel, Dread, Munchkin, Monopoly, or basketball.
The opinion that system doesn’t matter and people matter is very prevalent in the tabletop RPG community, and I think it is obviously for two reasons. One is that the community is so full of disgusting human begins, that a quality group is very rare. Even a mediocre play experience with decent people feels incredible when put in contrast with the median experience, which usually involves “that guy.” Choosing a more appropriate system may raise that game from mediocre to great, but players don’t care because mediocre feels great compared to what they usually have.
The other thing is that the most popular systems are all effectively identical. Most D&D editions, Pathfinder editions, GURPses, Werewolfs, 7th Seas, L5Rs, FATEs, and even Star Warses are not really fundamentally different systems. They use different dice. They have different settings, but they all have the same basic structures. It actually doesn’t matter what system you choose when games like those are the choices. A player who has tried those is probably right to think that RPG system choice does not matter.
But if you go ahead and play some actual different games, this is when the eye opening can occur. Is anyone going to seriously suggest that it doesn’t matter if you choose to play D&D or 1001 Nights? Is someone going to really suggest that it doesn’t matter whether you choose to play Ghost Court or L5R? That position is so preposterous as to not even be acknowledged. What next, it doesn’t matter if you watch Bambi or Ichi The Killer as long as you are watching the movie with a good group of friends?