Would it be ethically correctly to make a time-machine in order to go in the past and kill Adolf Hitler as a baby?
No, it would not be. The reason for this is that the baby Hitler was as yet innocent. He had not done anything that would have made him deserve to die.
To be ethical, we have to treat people as ends in themselves, as if we care about them for themselves. We cannot just treat them in ways that seem convenient to us. It would be convenient to kill baby Hitler, but it would not be an example of treating him as an end in himself.
If you are going to assume that you have a time machine, the ethical thing to do would be to go back and to start trying to mold young Hitler into a good and decent person. This would treat him as if he, as a person, mattered.
Baby Hitler would have been an innocent person and would not have deserved to die based on what we know he became. Killing him would be unethical.