Higher Attack increases your chance to hit, not your damage.
Higher Speed decreases the number of steps and actions enemies can make. It will also increase the duration of buffs/debuffs.
Higher Defense decreases the chance of getting hit by physical attacks, not damage reduction. Certain physical skills will ignore Defense and so will the legendarymaceMaelstorm. Defense is capped at 82%.
Higher Intellect decreases the cooldown on magic skills and affects the success rate of Combine.
Higher Faith decreases the cooldown on holy skills and buffs Holy Armor/Light thorns and Judgements healing.
Higher Magic Resist decreases the chance of getting hit by magic skills. It is capped at 75%.
Higher Stealth decreases the chance of enemies noticing you per turn. At 18, you have a 0% detection chance (except in the higher tiers of The Tower).
You CAN fight and kill Scavengers, but do so at your own risk as they can be very hard to fight for certain classes, in early levels, or if you didn't clear the level.
After the first kill, they will raise their prices.
After the second kill, they will no longer be friendly and will always know where you are while staying in stealth until they appear or use their skills. They become very deadly and high priority targets that have a very high chance to ruin your game.
Charming an aggressive/greedy scavenger will stop them fighting you.
When killed they will always drop gold or an random item or skill, be it useful or not.
Killing a scavenger will reward you with a high amount of experience points that count towards your next level, usually an entire level on the early stages.
A GREAT strategy for killing the scavenger is to use the Sleep Powder on him and then proceed to take him out with your weapon of choice! This works well with any weapon, as the Sleep Powder has a long duration, even longer than that of the Sleep skill.
The Scavenger, as a spellcaster, can be dispelled (and thus rendered sleeping), which is useful for a fast kill or to run away. The 2 Elementals he summons will also be killed when dispelled. If dispel is already recharged or you have an offering then you can stun the elementals to run away or fight.
A good tactic for killing scavengers is to use polymorph on them, this will only change their health and they will still hit like always and have/use the same abilities. But it is great for killing higher difficulty enemies such as the scavenger because it can lower the fight into a mere two moves.