Honestly, "spite and rage" might make an effective defense there. I cannot tell you how many times my uncle, enraged at me for doing things not-the-way-he-would-have-done-it, threatened to destroy something I had bought to do something my way. For example: LED light-bulbs. Yes, they have an up-front sticker-shock price, and he hated them over incandescent light bulbs, but the sensible thing, once they're already bought, it to just use them.

Instead he would rage, and scream "I'm gonna break these fuckin' things!" at me.

So, yeah, this guy could potentially defend himself by claiming to be so furious at his partner's profligate spending that, rather than attempt to offload the machine at a loss and recoup some of the money, or trying to find work they could undertake now they had an excavator, he got furious and destructive.

Would that work? Hahahah, who knows? But it might.