Why does the God of the Old testament seem so different from the God of the New Testament?

The God of the Old Testament is the same as the God of the New Testament- he was the same yesterday, today and forever. If you see Him being mean or unpleasant to those who don't obey his Laws in the Old Testament, you will see in the end that he is still mean to the sinners- he will cast them all in hell forever. But on the other hand, he blessed the obedient like Abraham, Isaac, Jacob and their descendants who are faithful.