The script was killed by the operating system. Negative return values are the signal number which was used to kill the process.
The script needed too much memory. I found this in syslog:
Out of memory: Kill process 26184 (python) score 439 or sacrifice child
Killed process 26184 (python) total-vm:628772kB, anon-rss:447660kB, file-rss:0kB