That’s because of the “Recreation Limit” which you can define in the Status Settings. This limit specifies the critical fitness value at which your creature is to weak for further attacks. If the Fitness value goes below this limit, the creature will stop its activities and wants to go home to recreate – by default the value is 20% but you can set this value to zero, so that your creature will fighting until it will die.
To compensate suffered damages and to recreate the full fitness you could define an idle behaviour as it is pictured in the following example. Add such a behaviour rule to the leisure activities of your creatures’ home location, so your creature can recover to be ready for its next fight, but this just as a suggestion, the settings allows you to define each kind of behaviour where ever you want, so you are absolutely free.