Couldn't he run as an independent? And basically screw the republican party?
Few people here actually addressed your point.
1) The Republican party is basically screwed anyways. There was a crack that formed in the Nixon administration (basically the GOP never fully came together as a whole when the southern Democrats joined starting in the late 1950s, but mostly throughout the 1960s and early 1970s). That crack was nearly healed under Reagan, but it didn't take. Note the loss of Bush Sr. when Perot ran and the formation of the tea party under Bush Jr. There is a full out civil war internally now.
Trump is the symptom, not the cause.
2) Trump can actually temporarily HELP the GOP by running as an independent. There is a very good likelihood that it would end up being ~45% Clinton (D), ~30% Trump (I), ~25% Cruz (R). Yes, that would lose the presidency. But, that could very likely give the GOP a near veto proof majority in the house and senate because Trump would bring in the tea party voters and Cruz would bring in the rest of the GOP. The GOP would dominate down-ticket in that election.
It would give the GOP the mandate (big house and senate gains) while Clinton would be crippled by getting far less than 50% of the vote.
True, that would only last 4 years, but that might be the time the GOP needs to fix their rift and find actual candidates that people want to vote for. Short of a miracle happening for the GOP between now and the end of the primary, Trump running as an independent might be the best outcome that the GOP can get from this election.