I am looking for a MARS (a software to run CoreWar warriors) working on GNU/Linux, but the only one I could find (nMars) crashes when I open or create a warrior file.
Yes. There is a variaton of pMars available for Linux. If you use Arch Linux, it is available in the AUR. For other distros, you could attempt to download the tarball and compile and install it manually.
For Ubuntu: pMARS is available from the official Ubuntu repositories from Ubuntu 19.10 (Eaon Ermine) onwards. Install by running
sudo apt-get install pmars.
For Debian: pMARS is currently in the "testing" distribution of Debian, so it might become available in stable Debian distributions starting from Debian 11 (Bullseye).
Alternatively, you could compile pMARS from source.