To use the language of privilege and such, a non-trans person saying "die trans scum" is offensive even if it is intended as a joke, since the person making the joke has the privilege of not facing the possibility of being killed for who they are. It makes light of a real struggle that a group deals with on a regular basis.

Within our society, white cis men are considered the default group which all other groups are defined in comparison. Being the default brings some privilege, but there are also negative consequences. For one, it is considered more acceptable to mock or portray negatively the default group than other groups. Everything from the use of racial/gender slurs to more general insults are considered more acceptable when they are directed at the default group. The message that is communicated to men is that they just need to accept this since hey, they clearly have things so much better anyway.

Now, being the butt of jokes and insults is not comparable to fearing for your life in order to feel like you aren't living a life that isn't yours. But what makes jokes like this offensive is you have a privileged person (not in default group) perpetuating or making a joke about the way in which they are privileged over the other group.

If you want to argue about who has it worse and whether it really matters whether that makes certain things okay or not is one thing. Personally, if you want to talk about equality and making things better by not doing inappropriate things, you shouldn't assume that you can get away with something you condemn in others because you don't know what life is like for them or how you may be using your privilege in a hypocritical way.