First of all, God the Father, God the Son, and God the Holy Spirit are not all the same person with a personality disorder. They are one by unity. They are of one mind and purpose, just as the church is supposed to be of one mind and purpose- does that make the church all one person? Of course not, that would be a personality disorder of the greatest magnitude lol. The Bible says we are to teach the same thing in all the congregations of the church.
Second, God seems different in the Old Testament because He did things physically then that He does spiritually now. David killing his tens of thousands seems much more drastic than tens of thousands dying to self in order to live for God. Taking adulterers outside the city to stone them to death seems more drastic than an adulterer taking their cross, dying daily to that sin in order to live for God.
These things seem more drastic, hands on, involved, responsive, etc. But they are the exact same things God is doing today spiritually instead of physically. He is preparing us to eventually leave the physical, and live in a spiritual realm with Him.