The best explanation/analogy regarding Pharaoh is noting how sun affects different materials: It melts wax, it hardens clay. With Pharaoh, we see an inclination within him to do the right thing, to free the Israelites. But the material liabilities (his own wealth and comfort) were also factoring into his decision. His love of self (clay) was greater than his love of others (wax). God fought hard for Pharaoh, but in the end it is always our own choice that God allows. Pharaoh could have joined in God's plans, but Pharaoh's own plans were always his priority.
We see this hardening of hearts in earthly matters as well. For example, the more part of the country claims President Trump has the best interests of the country at heart, the more the other part of the country hardens their hearts against him. The same man who melted the hearts of some, hardened the hearts of others.
Consider how many times, and over how many years, God worked with Pharaoh. God is patient, God is kind--and He has His own plans, His own work which will prevail. When His work does prevail...well look at the reaction of some of the people of Israel. They lamented their freedom, arguing they should have remained in Egypt!
No, God does not give us over to dark things. God remains love and goodness--it is who He is. It is the individual who turns from love and goodness (which softens our hearts) to favor material wealth and/or selfishness, self-interests (which hardens our hearts).