The threat a person poses is only one consideration. Justice requires punishment (btw, mercy only makes sense in light of punishment as justly deserved; you can

The threat a person poses is only one consideration. Justice requires punishment (btw, mercy only makes sense in light of punishment as justly deserved; you can only be merciful to someone who deserves punishment). If the crime committed is proportional with imprisonment, even if that person is not a threat, there is nothing unjust about that in principle. (The question of whether there are people in prison who ought not to be is a particular consideration not relevant here.) There are perhaps practical considerations that can affect the form the punishment may take, of course, but the point here is that threat posed is not the only or even the most essential consideration where just punishment is concerned.