On a side note, I rather doubt that the triangular bayonet blade runs afoul of the Hauge convention as it was used on several military rifles including both the Mosin Nagant and certain SKS variants well into the 1950's and 1960's.
I'm not sure about Czarist Russia, but I am fairly certain (without checking) that the Soviet Union was not a signatory to the Hague Convention, nor Communist China, the two major users and suppliers of the Mosin Nagant and SKS design rifles. They would have no qualms about a triangular bayonet (or anything else convered under the European "rules of warfare".
I think that if you used the bayonet attached to a rifle to repel boarders, when you had other opitions, its certainly going to used against you in court (should it go to court). How successful the prosecution is using that to tarnish your image in the eyes of the jury is questionable, but I'm certain they would try.
One thing that will almost surely be ask is why didn't you just shoot your attacker. Answering that "I didn't want to wake up the neighborhood" probably wouldn't get you any points from the jury, unless maybe you are in the right part of the country.
Remember that the underlying premise of a claim of self defense for the use of deadly force, is that you
had to do it. You had no other choice or option. You are admitting you did it, but that you had to do it. "He needed killin'" is no longer a valid legal defense in the US that I am familiar with. Although the sentement still abounds, and you might get agreement, it no longer excludes you from the penalties of law.
SO, you had to use the mounted bayonet to defend yourself. And the bad guy got hurt/died. Other than the scenario that you are surpised and have to grab the closest thing, and it happens to be an unloaded rifle with a bayonet affixed (and unsheathed - if you take the time to take the sheath off the bayonet, that opens up a whole 'nother kettle of fish as to the content of your character) just how do you explain to the authorities (police/prosecutor) just why you had to use it.
Personally, I'd love to hear that. And your lawyer ought to hear it first, then after he tells you that you were an idiot, you can try and figure out a way to keep the law from charging you.
For some odd reason, I'm, getting the mental image of an aged Pacific war vet, have a bad flashback, grabbing the display Arisaka (because the Garand, which IS loaded, is in the bedroom

) and lunging on the gangbanger who broke in, screaming "Banzai this punk!!!!" And, of course, leaving the punk pinned to the wall until the police arrive, so as to not disturb the evidence. OR perhaps sitting on the porch, having a smoke cleaning the blood off the bayonet when the cops show up....
Either way, that's going to be a tough sell in court. The idea of using the bayonet is just so ..farfetched.. today it will boggle the jurors minds.
OK, you bayonetted him, because the rifle wasn't loaded, right? Why didn't you buttstroke him, instead? Why did you have to stab him? (and there better be only one stab wound, or you are REALLY going to have some explaining to do! And no matter what the truth is, it might not help!
good luck