Genesis 6:5 cannot be ripped out of context from verses 1-4, which clearly tell of the ‘sons of god’ and nephilim. What they introduced to the planet, and to man in particular, was God brought about the flood.

Similarly, the verse in Genesis 8 has to be seen in light of what has just happened. Though Noah was ‘perfect in his generations’ ie had no ‘sons of god’ genetic issues, man was still wicked in God’s sight because of the fall of Adam, the fall that leads to all men being sinful, and all needing a redeemer.

The altar in the OT is a foreshadow of the cross, and the animals sacrificed a shadow of the perfect sacrifice that was to come. It wasn’t the smell of dead burning animals that God desired, it was why the sacrifices were being made, and the intent of men in doing so. That is why God later chastises rebellious Israel for their sacrifices which were nothing more than ritual, while they continued to worship idols.

Even after the flood, the path to redemption for man could only come from the one promised in Genesis 3. That redeemer was/is Jesus Christ.

There is no contradiction in the two verses quoted by the op, not when you read them in context of the book in which they appear.