Hi,
I think that's because you're using if-else statement.
If skill1 is equal or greater than 0 then other ifs are not executed...
Try this way:
if skill1 < 0:
skill1 = 0
if skill2 < 0:
skill2 = 0
if strength1 <= 0:
print ("%s is dead") % (character1)
if strength2 <= 0:
print ("%s is dead") % (character2)
Here you have link to if-else in python language:
http://www.tutorialspoint.com/python/python_if_else.htm[
^]
Hope it help you.