The Freedoom medikit sprites (including the stimpack and berserk pack) have a red cross emblem on their side. The red cross is a humanitarian symbol recognised by and protected under the Geneva convention. Laws in most countries restrict its use, and the Canadian Red Cross has actually spoken out against the use of the symbol in video games. Real medical kits don't actually use a red cross for the same reason.

Suggestions for how to fix the problem: Remove the cross
Change to a different color, eg. green
Change to a different symbol, eg. a red 'H' Preemptive answers to potential objections: It's not a big deal

Admittedly it's unlikely the project would be sued by the Red Cross; if they were to sue anyone then it would more likely be a commercial games company. However, the red cross is an internationally protected symbol and as I understand it, using it in this way may be illegal, at least in some countries.

Doom did it / other games do it / it's the standard symbol for health in video games

The fact that other games do it does not make it legal or legitimate.