Yes, because there are patterns and they usually admit it.

Anyway, its not like you kill a woman then get inmediately accussed of sexism. Its you kill a woman -> you are investigated -> the investigation determines if it was due to sexism or another reason.

Now when its a male who kills his woman, it usually happens because there is already an abussive behaviour and he tends to use the force against her until It went too bad.

Thats a extremely weird case and a minority. Im pretty sure it will be considered a revenge crime.

Now, the case of a woman getting killed by her ex partner, you need to take into consideration It is considered sexist because It is not an isolated case, but a trend. A trend caused by an archaic thought that men can control what women should do. Thats the difference, a woman killing her ex-boyfriend is an hypotethical case, while a man killing his ex-girlfriend is a common case related with hateful behaviour towards women.

If in a future the oppossite case of women killing men in masse happens and is bound to hate, then it will also need to be considered sexist.

The whole fact that a guy got the idea that is ok to get revenge for "leaving him" is sexist dude.