Why is God portrayed so often in the Old Testament as a cruel tyrant? Cursing and dooming people? I don't understand and I don't want to serve a cruel God, but maybe I'm missing something...
The reason people see cruelty in the Old Testament and love in the New Testament is because that's what they want to see and that's what they've been taught to see. Here are a few examples of what the Old Testament says about God (all quotes from KJV):
Thou in thy mercy hast led forth the people which thou hast redeemed: thou hast guided them in thy strength unto thy holy habitation. (Ex. 15:13)
O give thanks unto the LORD; for he is good; for his mercy endureth for ever. (I Chron. 16:34)
Hear me, O LORD; for thy lovingkindness is good: turn unto me according to the multitude of thy tender mercies. (Ps. 69:16)
I will mention the lovingkindnesses of the LORD, and the praises of the LORD, according to all that the LORD hath bestowed on us, and the great goodness toward the house of Israel, which he hath bestowed on them according to his mercies, and according to the multitude of his lovingkindnesses. (Is. 63:7)
And here are some verses from the New Testament:
Then said he to the multitude that came forth to be baptized of him, O generation of vipers, who hath warned you to flee from the wrath to come? (Luke 3:7)
Therefore shall her plagues come in one day, death, and mourning, and famine; and she shall be utterly burned with fire: for strong is the Lord God who judgeth her. (Rev. 18:8)
And thus I saw the horses in the vision, and them that sat on them, having breastplates of fire, and of jacinth, and brimstone: and the heads of the horses were as the heads of lions; and out of their mouths issued fire and smoke and brimstone. By these three was the third part of men killed, by the fire, and by the smoke, and by the brimstone, which issued out of their mouths. (Rev. 9:17-18)
And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell. (Matt. 10:28)
The Old Testament says a lot about God's love and mercy, while the New Testament also speaks a lot about His righteous wrath and judgement. People just see what they want to see. Jesus said "Seek and ye shall find". If you go through the Bible looking for God's wrath, then you will find it but, if you look for God's love, mercy and grace, then you will also find that, both in the Old and New Testaments.