Here's why:
* Pentangeli's fate is ambiguous. The film implies Pentangeli was killed, but it's never explicitly stated. Some believe he was killed by Michael's orders, while others believe he was killed by Hyman Roth or someone else.
* Michael's motives are unclear. Michael is shown to be increasingly isolated and paranoid by this point, and he's distrustful of Pentangeli, who's been acting suspiciously. But the film doesn't show him directly ordering the hit.
* The film deliberately leaves the question open. The filmmakers intended to leave the audience wondering about Pentangeli's fate, adding to the film's ambiguity and complexity.
While it's possible that Michael may have ordered the hit, the film doesn't provide concrete evidence to support that claim. It's ultimately up to individual interpretation.