Studied far too much law stuff in uni. Insanity is very, VERY rarely a good defense. Hard to get. I wouldn't count on it. People tend to ignore it anyway, if what you did is bad enough, as in they'd rather choose to crucify you. As for being a minor… you could easily be tried as an adult if bad enough. Then again, if it's just a neighbour's pet, not likely to be a big deal, if you make it look like an animal. Look up the Slenderman girls who stabbed their friend. Absolutely insane and minors, tried as adults and not considered insanity.
Also, don't do it, man. That's a new level of depraved. Also, it's not worth it.
Plus - if you do worse shit in life, they're more likely to use this against you to show how terrible you are. Just brings bad news whatever way you spin it.