And of course both of these are different from, say, the CN bard who visits town, or the CE rogue. Slaadi and demons are alien -- from another reality. They drive to more extreme examples of these alignments than most mortals express. Part of the thing you're dealing with when you deal with anything extraplanar is that they are not overly concerned with mortal problems. Perhaps the celestials take mortal concerns into account and try to do little damage, but even an angel is a dangerous creature whose logic is foreign to the concerns of an LG person just trying to live their life.
I think it might also be good to think of what a being of LN essence might do in this situation.
If such a being finds themselves in a town in the Prime Material they might go about creating absolute order. Using the market example they might impose their will to make it an ordered place. No crowds, only lines, everyone walking a designated pace, prices set, etc. Attempting to remove individuality from the town, to turn it into a clockwork machine under threat of force.
That reads just as evil to us, but the intent is different and punishment only comes as a result of acting 'chaotically' which in this case means having your own thoughts.
Slaad are the opposite of this.