aren't plasma weapons just in general extremely tempermental and prone to blowing up by chance? of course a bad operator would exacerbate the issue but i'm pretty sure they're just unreliable turds that practically nobody except the people who made it understand, so that if one does malfunctions it's not the operators fault.
i mean stuff they can still malfunction in the hands of a space marine vanguard so i assume it's just not a "bad operator" issue.
"Gets hot" is never explicitly stated how it comes into play, but a common theory behind it harming the operator is due to extended use requiring a rapid venting of excess heat. Which is why the penalty is armor-savable and allows you to continue using the weapon if you pass the save, or even if you fail it if the character has multiple wounds. It's like if using the plasma rifle/pistol in Halo damaged your health whenever you allowed it to overheat. If the gun really exploded then you either wouldn't be able to keep using it or "gets hot" would be instant death.
Ultimately it's just a method of balancing the weapon on the tabletop, because it is a very cheap and powerful killing tool. A plasma gun is Strength 7, AP 2 and can fire twice if you don't move. That's potentially two 50 point Terminators killed by a 7 point guardsman and his 15 point gun in a single shooting phase, it NEEDS something to hold it back so that it's not a complete no-brainer to take. No, a fluff space marine would probably not be inattentive enough to allow his weapon to overheat, but then again a fluff space marine also would never be able to be killed by a single guardsman with a bayonet, but both of these things can happen on the table.
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