Nobody mentioned legal or illegal. Everything u/lookcloserlenny said is absolutely true. Even fruit fly research, pests they might be, there are committees and boards that must authorize all animal research prior to the animal procurement, the source of the animals matters, the housing and care of the animals matters, etc.

If you break any rules your institution has, like if you kill your mice using approved methods for the institution but not approved for your particular grant (eg you must suffocate the mice with CO2 and then break their neck after that, but other people can just use a guillotine in your building) your professor could lose their privilege of storing their animals in your vivarium. No laws have been broken, nobody goes to jail, nobody pays a fine. Well, there are no fines, but you lose your grants immediately, because you were being paid by the NIH or NSF or what have you, to study mice, and you're not doing that anymore, so now you can't very well collect the rest of the monthly portion of the grant that funded your stipend or salary, can you? No. So you aren't even fired, you don't lose your job, you still have every opportunity to apply for other grants, but the grants you were working on may be frozen, and really, practically, you're entire lab is immediately effectively fired, but no, nobody pays any fines or goes to jail if one grad student was using the lab next door's guillotine because your lab ran out of CO2, the student thought it was faster or easier than borrowing a CO2 tank, and the professor finds it all out when it is too late, but was ultimately responsible.

Yeah, its not illegal, though. I mean, its not even against university policy. Yeah, any lab with federal funding or at a public uni can just treat the animals all willy nilly.