Why is it that the devil gets in the mind of the weak?

For the same reason "god" is.

The weak minded need to believe in something more powerful than humanity, that they can blame for problems - rather than taking action.

Because the weak will always look for excuses. "The Devil made me do it" or "it was God's will" are both cop-outs for failing to take responsibility for something.

It's not just the weak---it's the nature of us humans.

Read Romans 7.

A tad over the top perhaps, but here's a description of him:

Satan is not some mythical character with horns, tail, and pitchfork. He is very sophisticated and specialized. He is the greatest under-cover agent of all times., Biblical descriptions of him include everything from an angel of light to a ravenous beast. Anyone is a welcome challenge to him, from Adam to Jesus himself. Nobody is too good or powerful for his advances. He was strong enough to defeat one of the apostles of Jesus.

Ravenous beast and sophisticated and specialized are how CS Lewis portrayed him in his book "The Screwtape Letters."  

In Judaism Satan is an angel/messenger of God, (Job 1:7) (1 chron 21:1) his job is to test mankind.

Isaiah 45:7 shows that God creates all things both good and evil.

For we were granted freewill to choose between "good and evil."  It was only when we were given choice that sin entered the world. Satan is described as the serpent (trickster),  the angel of death (he's there at the death sentence), the accuser (Zech 3:1) ego, and arrogance. 

Hence, we are in a continous battle to either give into sin, lust, greed, gluttony, envy, pride, sloth, and wrath, to give into worldy desires and those of the flesh, or proceed toward a spiritual godly way of life which takes a person closer to  God. Satan can also influence other's in getting you to commit sin, just as the ego does. Satan is seen as a sort of purifyer. Jews do not believe Jesus was the Messiah. They see no need for a Messiah because mankind is not beholden to the original sin doctrine. They believe that man can behave honourably by following Judaism. 

http://www.beingjewish.com/basics/satan.html

However, in Christianity a Christian when reborn (a new mind, new person). Becomes filled with the Holy Spirit, which acts as a defense against Satan who is seen more as an out there figure, a fallen angel. Jews see Christianity as a different religion altogether.

However, some Jews believe all religious text leads to a similar goal.

I think the evil/bad (the physical present of the devil is NOT inside men/women) is in peoples mind and their actions he sees and influences them to continue their bad/evil ways and habits.

Nonexistant.

Hello
Megan! Weak people are often helpless and this makes it pretty easy for the
devil or anything that can reassure them, to get in their minds. It will be
wise to avoid exploiting, but helping such souls! Guiding them towards paths
that will strengthen and awaken them will be the right thing to do!