"The earth was spoiled in God's sight" or "God saw that the earth had become very bad." Possible meanings are 1) all the people on the earth were very bad or 2) everything on earth had become very bad.
This can be translated as "and the earth was filled with violent people" or "and violent people were all over the earth" or "because it was full of people who were acting violently toward each other."
## all flesh ##
This refers to human beings.
## had corrupted their way ##
This means "they stopped living the way God wanted" or "they behaved in an evil way" (UDB).
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