Actually you can. If you are legally insane you are no longer responsible for your actions. But you still have human rights.

On topic:

There is a very interesting underlying philosophical problem to that discussion. Remember that not to long ago, e.g. Jews and Africans were considered "sub human". In Nazi Germany, murder was still a crime and you couldn't just go around and kill 6 mio people. But murder, by definition requires the killing of a human. But luckily those damn jews were "sub-human" and therefore killing them was not murder. And before that Africans were considered to be only somewhat better apes. Hence you could write a constitution that makes all man free, but not "ape like negros".

Today we luckily know that this was sick and wrong. But the basic question remains and has somewhat shifted. Currently the "hotspots" of this discussion are embryos and brain dead comatose patients. In the future it might be clones are gentically engineered "harvest bodys" for organs.

This is why one must be very careful with saying stuff like "but those obviously aren't humans", because the question shouldn't be "are we giving them rights", but "are we taking rights away from them"

The highest value in the constitution of Germany is "human dignity", it reigns above all. But there is no definition of "human dignity" (or "human" for that matter), because then you would have to draw a line, and whenever you draw a line, there is something "outside" of that line. And that is not allowed to happen (again).

Therefore I'm also very much against applying "human rights" on dolphins, because that might "water down" human rights. And I still think that all humans are still special and different from mamals. And, to be honest, despite everything I've said before, I'm pretty sure I can say "dolphins aren't humans". But that notwithstanding, we should reconsider how we treat those animals in the future.