striktly speaking, yes, mutulating a corpse, especially for such a trivial thing like a potential warning is against a good alignment.

its obviously not enough to make a alignment shift, and other people might do little more then raise a eyebrow at such a act of cruelty, but it has several implications.

first of all, that bandit likely had family/friends who are completly innocent but still loved him. by pinning him up there, you MIGHT scare off other bandits, but your sure to inflict some minor trauma on any loved one of him who saw him.

also, a corpse is a unhealthy thing, hanging it up is a danger for any innocent passing by there.

and lastly, yes the bandit might possibly have been a murderer. but lets be fair, you dont know that for sure, and all he did to you was defend himself when he got attacked. making a signpost out of the guy is just very disrespectful.

all in all, its a neutral act at best, and a evil act in most times, even tough its only a minor one. its certainly cruel, and it should get at least a "what the hell" out of other good aligned people