Edit: In the real world, this is more likely to happen by accident than intent.

No regulations exist yet.

The offender could claim whatever mod he made wasn't intended to cause harm to others. Then you have to prove intent, which can be difficult.

Or you have to have laws which cover acts done with mental states short of intent to cause harm like (in ascending I order of seriousness in existing law—and all of these are already covered for acts that get people killed) criminal negligence, recklessness, and depraved indifference to human life.

Of course, it's a lot less difficult to convince even a criminal jury of intent than people in HN often seem to think; legal proof, even to meet the “beyond a reasonable doubt” standard, is far less than logical proof.