MC is right in his speech
The founders of the nobles houses were great people, and their decendants use the actions of their ancestors, without doing anything themselves.
Just because your great-grandpa killed a dragon, does in NO SHAPE OR FORM mean that you can take credit for it.
If you get a "random idea", make a small portion of it, and taste it, so you don't ruin the whole dish if it fails, and if it's succesful/tasty, you can make it like that next time.
The whole "she just came and attacked him without reason" is far FAR *FAR* too little information, there must be SOME reason why she did it (will most likely be revealed later)
My theory is it was a doubleganger, to blame her or he was secretly a bad guy.
Workplaces like that, should hire 1-2 people to clean and such, to ensure it dosen't pile up.
The easiest way to clean up, is to prevent it to get too dirty to begin with.
In a way, he reminds me of Goblin Slayer, he keeps to the smaller pests, partly because most others don't want to.
I have a personal saying: "Someone needs to take out the trash, and if nobody else wants to, then I might as well do it"
So he got EXP for killing the masmascal with poison, and then got EXP for the dead monster who died from eating the poison filled masmascal.
That's one way to chain the kills.