He did not set out to hurt her. He probably passed by a dozen objects far more capable of causing harm than a hose. Pens, staplers, frames for art, a statue, hell even his fists are all more harmful. I struggle to think of something that causes less harm than tap-pressure water. I would say he explicitly set out trying not to cause any harm, or he would've reached for a more obvious and harmful option.
Neither of us knows what his state of mind was, but I don't see anything outright implying contempt. Doing something bad to someone doesn't require contempt, it can be a practical means to an end. Based on him calling social services, I would say he didn't have contempt, he just realized the government wasn't going to help and took it into his owns. Similar to managers when they have to fire someone; doing so doesn't imply or require contempt, it's just a means to an end.
Well he did, it's the only reason he was arrested.
How would you feel about a stranger purposely and repeatedly drenching your sick grandmother or elderly immuno-vulnerable mother with a water hose in the middle of winter?
I promise you I wouldn't. My neighbor's sprinklers hit me not infrequently (they hit parts of the sidewalk). I've had a drink thrown on me in anger. It is absolutely not intrusive or worthy of prosecution. It's annoying, but there's no permanent harm other than a bit of resentment that it happened.
None of it even comes close to the line of being worth having armed men show up to lock the other person in a cell. That whole process is deeply intrusive and violates personal sovereignty to a far higher level than getting wet ever could.
I think we may just fundamentally disagree on the level to which the government should interfere in interpersonal issues. My view is that anything that lacks some form of permanent harm (be it bodily, financial or emotional) doesn't require the government's involvement. Two adults quibbling and annoying each other doesn't rise to that level. Someone throwing a drink, or spraying a hose, or playing a song the other person really hates just doesn't meet that bar and doesn't require sending anyone to jail. It's a silly use of government time and resources, and it feels like a waste of my tax dollars.