Well the Sligs are merely the pawns of the Glukkons. So they are not really evil, they are the tools of the actual evil dudes. It's a bit like calling attack dogs evil, when it's actually the people who use the attack dogs against innocent people who are evil. Or even like Orcs in LOTR/Hobbit. Without Sauron and Saruman controlling them and putting them to mass evil use Orcs are just a tribe of creatures who are a bit aggro, but are of no particular threat to the various other races in middle earth.
I actually prefer not to kill the henchmen if I can get around them. But it seems most games are designed to make you kill the henchmen.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix
