Leave old kernels installed could be dangerous?

You do not have to leave old kernels, and you do not have to remove them.

Old kernels just take space of your HDD. That's it. You can have one or two old kernels installed just in case you want to boot with one of them.

Old kernels do not affect a working system in any way.

So it is 100% SAFE to leave old kernels installed if this was the question. They will not be used unless you deliberately boot with one of them.

The only problem you get by keeping all the old kernels is space. Every kernel-image along with the headers takes about 300 MB.

Eventually this can fill the partition and make security updates impossible.

You always want to keep at least one older kernel to have a fallback just in case.