Killing someone does not make his stuff yours.
In our Iga, a man killed in a duel or in self-defence still leaves possessions claimed by his household or lord. Taking his purse can be theft, even when the killing was justified.
You can keep weapons awarded as an agreed prize, bounty or authorised spoils. Bandits’ stolen goods still have owners.
Changing a sword’s fittings or mon does not remove those claims. To make it yours, obtain a recognised gift, purchase or award.
That fine blade taken from a robber might serve you for months before someone says, “That was my father’s sword.” Now you have something to explain.
And of course as they say, posession is 9 tenths of the law.