What might have influenced Cassius to accept Pindarus’s report?
Cassius has had a complete character shift since the beginning of the play. Previously, he was bold, defiant of the gods and omens, and confident he knew what was right for Rome.
In Act Four, the reader can see that Cassius's boldness and bravery are wavering. He resorts to tactics like guilt and pity to make Brutus feel sorry for him when they are fighting He is no longer convincing Brutus to follow him, but begging Brutus to like him and allow him to be his equal.
Cassius also doesn't agree with Brutus' plan to march to Philippi. Cassius would rather wait for the opposing army to meet them in Sardis. Again, Cassius is overruled by Brutus.
In the beginning of Act Five, Cassius is anxious and nervous regarding the battle. He has seen scavenger birds (like vultures) replace eagles, and he takes this as a bad omen. He and Brutus also say goodbye, in case they never meet again.
Cassius is in utter defeat when he receives Pindarus's report. Men have deserted him and even his flag bearer was killed. In such a negative state of mind, it only makes sense that Cassius takes Pindarus' report as truth. It also gives Cassius a reason to give up (commit suicide) without looking cowardly. Since he feels the entire battle is lost, he would rather kill himself than be taken as a slave by Antony and Octavius.
I believe that Cassius does “accept” the news that Pindarus gives him. In Act 5 Scene 3, Cassius orders Pindarus to climb a hill to see what is going on in the battle. Pindarus reports that the enemy has surrounded Titinius and then “ta’en” him (30-33). Cassius asks Pindarus to “come down,” and then moans the fact that his good friend Titinius has been “ta’en before my face.” After freeing Pindarus (for he is his slave), Cassius asks him to run a sword through him (40-50). It would seem to me that Cassius very much “accepts” the reality of Pindarus’s report, so much so that he realizes this news signifies that his own actions—killing Caesar and then battling Antony and Octavius—have met their inevitable end. Rather than accept defeat at the hands of his enemy, Cassius asks his slave, whom he had taken in battle years ago, to kill him, thus controlling his life rather than allowing his enemy to win in this way.