I think I just found a dead brown recluse spider in my shoe. Why are they so bad?

Poison. Brown recluse spiders possess a toxin that is extremely powerful and will cause your skin to die. Left untreated the toxin can reach the point where you may lose a digit or will be left with a very large wound. There is a specific antibiotic that actually helps neutralize the toxin. Seek help if you are bitten. Check with an exterminator if you think you have a brown recluse problem.
See below. Due to the toxin that they carry if bitten it can do severe tissue damage around the area that is bitten.