las,
Asides that although it is a question, but you can not hold that against Shimon and Levi to that extent, as after all they did follow the simple halcacha that was required so the mitzvah should protect them. We do find many hold that you are permitted to put yourself in danger to be mekayom a mitzvah, (as beg. of Pesachim by Shmuel and bedikah).
In any case, the point is that the Meiri seems to uphold that punishing by death sentencing is not an absolute Chiyov on B”D or anyone else. The mitzvah of ‘establishing courts’ are to enforce it in any way they seem fit, including death penalties.