Killing 50% of any one people is genocide, right? For example, the Nazis killed up to 50% of European Romani people and it is classified as a genocide.

Let’s assume killing 50% of n peoples is genocide.

Since killing 50% of n peoples is genocide, killing 50% of n+1 peoples must also be genocide, else a number N would exist such that killing 50% of N - 1 peoples is genocide but killing 50% of N peoples is not. The existence of such a number N would be quite contradictory, as it would imply one could undo genocide by killing more people. Additionally, if one were to first kill 50% of N - 1 people and then kill 50% of one more people some time later, both events would be classified as genocide, since killing 50% of one people is assumed to be genocide.

Therefore, Thanos did in fact commit genocide.