How could Kazan's testimony to the House Committee on Un-American Activities affect him negatively?

From Wikipedia

In April 1952, the Committee called on Kazan, under oath, to identify Communists from that period 16 years earlier. Kazan initially refused to provide names, but eventually named eight former Group Theater members who he said had been Communists: Clifford Odets, J. Edward Bromberg, Lewis Leverett, Morris Carnovsky, Phoebe Brand, Tony Kraber, Ted Wellman, and Paula Miller, who later married Lee Strasberg. He testified that Odets quit the party at the same time that he did. All the persons named were already known to HUAC, however. The move cost Kazan many friends within the film industry, including playwright Arthur Miller.

and

A turning point in Kazan's career came with his testimony as a witness before the House Committee on Un-American Activities in 1952 at the time of the Hollywood blacklist, which brought him strong negative reactions from many liberal friends and colleagues.

His testimony helped end the careers of former acting colleagues Morris Carnovsky and Art Smith, along with ending the work of playwright Clifford Odets.

Kazan later justified his act by saying he took "only the more tolerable of two alternatives that were either way painful and wrong."

Nearly a half-century later, his anti-Communist testimony continued to cause controversy. When Kazan was awarded an honorary Oscar in 1999, dozens of actors chose not to applaud as 250 demonstrators picketed the event.

Basically, for whatever reason, Kazan chose to name names to the HUAC which ended the careers of a number of people.

That's enough to get you shunned in a close-knit (almost incestuous) community like Hollywood in the 50's.

His friends and colleagues would certainly have felt betrayed and somewhat suspect about what he might go on to say about them if he was called again.