Creatures can have two types of alignments, Passive and Aggressive. A creature's alignment can be found in the tooltip at the top of the screen when the cursor is placed on them. Aggressive creatures will always attack players that step into the one of their four adjacent squares unless they have a Combat level 21 levels above their own. Passive creatures will only retaliate if attacked first.
Creatures in Party Dungeons will always be aggressive regardless of their tooltip.