The shorts don't want Tesla to fail. The shorts don't even think Tesla will fail.
The shorts just think the market cap of Tesla is insane. It's higher than much larger, profitable automakers who can provably make and sell cars at scale.
For Tesla to be worth its share price it has to grow by two orders of magnitude. I won't even say that that won't happen. It could. But it'll take years. And when it does the stock won't be worth more than it is now.
On top of that there are risk factors. What if energy density of batteries doesn't substantially improve and all electric vehicles continue to be unappealing to large chunks of the population due to range anxiety? What if all electric does win (it's going to eventually) but the other companies are able to compete well enough and Tesla just becomes another Ford?
Their share price is just out of control. The market can stay irrational longer than I can stay solvent, but I can totally see how someone with very deep pockets could find shorting Tesla a good bet even if they don't want it to (or even think it will) fail.