Also if it is the sons of Seth and they are sons of God why would they mingle with the wicked.
This precisely the point. The marrying of the descendents of Seth with the descendents of Cain resulted in the moral decline of humanity. As chapter six progresses we see that only one man was still recognized as righteous among all humanity.
Also if it the sons of Seth then why did it produce people that were of old,people of reknown,and were terribly wicked,which sounds like it produced people that were not on earth until the time of Adam until then,which sounds like there were people before Adam and Eve and the fallen angels disrupted them.
This was simply a matter of human genetics, nothing more. There is nothing in scripture to suggest that angels can reproduce after any fashion much less reproduce the human gene code. In fact, Jesus tells us that angels neither marry nor are they given in marriage. Since this is true it is quite contradictory to read Gen. 6:2 and draw a conclusion that angels did precisely what Jesus said they do not do. The term sons of God must be understood according to the pattern of its usage in scripture. Every other time this phrase is used (with one possible exception) it refers to man. That being true, by what rule of interpretation does one assign to this term some other definition - angels?
It could be that the fallen angels possessed people and that is why they were terribly wicked
There is nothing in the text to even suggest this notion.