Why do authoritarian regimes bother keeping opposition leaders alive in prison?

Some examples of politicians who spent a long time behind bars before coming to power include Vladimir Lenin, Vaclav Havel, Nelson Mandela, Aung San Suu Kyi. It therefore seems like a risky move for an authoritarian regime to keep their opponents alive in prison, rather than silencing them forever as soon as they come to prominence.

So why does e.g. Russia bother keeping Navalny alive in prison? They did try to assassinate him covertly previously but now that he’s in prison, why don’t they finish the job? What’s the incentive for keeping people like him alive?