Some squirrels show aggressive behavior because although they may look cute and cuddly they are still a wild animal. You shouldn't feed or try to pet a squirrel, because they may bite and in some cases may carry rabies.
It is always good to avoid an agressive squirrel, as it could be rabid. However, it may also depend on the type of squirrel. Red squirrels tend to be much more aggressive.
They can carry rabies, which is dangerous. Aggressive behavior isn't normal. Keep your distance. Where they are fed constantly, could have lost their shyness but still should not be aggressive.
Squirrels can most defiantly be dangerous. The squirrel is not a social animal and they are extreemly territorial. They will attack anything they consider both a threat to their territory and something they can beat.
