Then the subject is falling backwards. Plus, the tilt focuses the subject's view to the lower right. Lends action, what is he looking at? PLUS, levelling the horizon reduces the subject's importance.
(Jesus, I sound like an art critic. But hell, I think the pic is near perfect.)
it is not. a tilted horizon is never acceptable regardless of whatever else is going on in the photo. However the subject was standing with a flat horizon is authentic. The subject's actual stance is more interesting than the false stance that the tilting has inferred.
Also while obviously nobody should treat compositional "rules" as actual hard-and-fast rules, I do quite like this one if it is cropped to put the person and the horizon on the thirds lines
I've taken many photographs with non horizontal horizons. When the composition is more important than documentation, you can rotate the horizon any way you like
the level of the horizon is a key part of composition. it effects comfort, balance, and groundedness. when the horizon is not level it will feel disorienting, dizzy, or chaotic. yes, you can break compositional rules for artistic effect, but you need to learn the rules and why they matter before you can do so effectively. the example you posted below doesn't really make your case. it's not that great of a photo, rotated or not. to intentionally rotate the horizon to give it an uncomfortable or disorienting feeling is fine if that's the goal hell, maybe it's more to feel otherworldly or any other number of things you can derive from it. the point is that you need a reason and intent behind the unlevel horizon. what feeling were you trying to invoke by not having the ground beneath the feet of the viewer?
I remember my mentor told me, I forgot where its from, photography is made of two things, hours of setup for 1 good photo, and the once in a lifetime photos taken spur of the moment. Always know how to prepare and always be ready.
This is a lovely photo, best of luck on many more!