He was responsible for killing the league. The USFL played in the Spring but Trump insisted on going head to head with the NFL in the fall so after 3 seasons, the league decided to move to the fall. They ended up never playing a single game in the fall and the league folded.
The USFL sued the NFL in 1985 claiming the NFL was acting as a monopoly.
The trial received national attention, and Trump testified on June 23. At one point during his testimony, the legal team for the NFL stated that Trump had paid bussers and waiters to spy on NFL owners while they were staying at his hotels, which he denied.
During the trial, Trump stated that in March 1984, he had met with Rozelle at The Pierre hotel and, during the meeting, Rozelle tried to talk Trump out of moving the USFL to the fall, offering him an NFL franchise if he cooperated. However, Rozelle denied making this offer. Rozelle stated that Trump had been the one who set up the meeting and rented the space at The Pierre and from meeting notes showed that Trump had requested an NFL expansion team in New York City, with plans to sell off the Generals. Leslie Schupak, a person who was present at the meeting, later told Pearlman that Rozelle had told Trump, "As long as I or my heirs are involved in the NFL, you will never be a franchise owner in the league".
The Jury ruled against the NFL, in favor of the USFL but awarded the USFL damages of just one dollar. That was later tripled to three dollars. ESPN did an excellent documentary on the USFL and it's demise called "Small Potatoes: Who Killed the USFL?". The film makers left no doubt on who was to blame for the league's failure: Trump.
I can't imagine the NFL would ever agree to allowing Trump to own a franchise..
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