I would agree that freedom of the will is an important factor, and that man's fall plays a significant rle in why evil is in the world (Rom. 5:12), but I do not agree that God would leave Satan and demons out of the discussion when answering a "why" question on evil.
Matt. 13:24-28
24 Another parable put he forth unto them, saying, The kingdom of heaven is likened unto a man which sowed good seed in his field:
25 But while men slept, his enemy came and sowed tares among the wheat, and went his way.
26 But when the blade was sprung up, and brought forth fruit, then appeared the tares also.
27 So the servants of the householder came and said unto him, Sir, didst not thou sow good seed in thy field? from whence then hath it tares?
28 He said unto them, An enemy hath done this. The servants said unto him, Wilt thou then that we go and gather them up?
In verse 39 Jesus says, "The enemy that sowed them is the devil." So yes, in a sense God would say, "no it's Satan's fault." Many who have been allowed to travel in the spirit realm or were given visions of the spirit realm were allowed to see spirits of perversion and other ways that evil spirits were working in the world. Along with Scripture, God has attempted to show His church that demonic forces are ruling the darkness in this world. Our warfare is not against flesh and blood but against the rulers of this dark age (Eph. 6:10-18).
There is a need to emphasize Satan's role because the Bible emphasizes Satan's role. I have given you a few Bible references for this but there are more. The evils that Paul suffered that he describes in 2 Cor. 11 are said to have been done by a "messenger of Satan" (2 Cor. 12). The afflictions that Christians suffer in this world are because the devil walks about as a roaring lion seeking whom he may devour (1 Pet. 5:8-10). Paul said that the reason he could not visit the Thessalonians was because Satan hindered him (1 Thessalonians 2:18). The reason that Jesus gave for a woman being in bondage to sickness was because Satan kept her bound (Luke 13:16). Peter said that Jesus primary purpose on earth was to heal all who were oppressed of the devil (Acts 10:38). Look at the gospels and see how often Jesus casted out devils, crediting them for the evil that men suffered.
These are just for starters. We can go into Job, David's temptation, Adam and Eve, Cain, Simon the sorcerer and many other areas. Leaving Satan out of any response to the problem of evil is simply not a Biblical response to evil.