I believe you are experiencing this because your default shell is dash (the debian almquist shell), but you are using bash syntax. You can specify bash in the shebang line with something like,
#!/usr/bin/env bash
Or, you could use the dash and bash compatible back-tick expression suggested by admdrew in the comments
kill -9 `cat /var/run/myProcess.pid`
Regardless, you can’t rely on /bin/sh to be bash.