In the old school of astrology both Mars and Saturn regarded as "evil", and that is where a lot of it comes from.
Today we know that it is how the person handles the energy that is the key, and no planet is "evil".
The key is to look at the energies of the 2 planets - Mars is action and Saturn is limits - so the natural thing would be to say is that Saturn is putting the brakes on Mars. Is that "bad" - not necessarily so though, for it may enable a person to do things well thought out and with a great deal of structure - in other words, there will be less impulsiveness in action.
Dave