Frenzy's death and Rumble's fate. Soundwave's reaction is just...heartbreaking.
The aftermath. The others cassettes are also completely devastated. Not only this but Rumble had been reformatted meaning that even though he comes back, he wouldn't be the same anymore and would be a very different person. That's the reason why Soundwave refuses to wake him up and is only does because the cassettes force him.
Soundwave's capture. At this point, you can see just how he crossed the Despair Event Horizon. He has no more reason to live.
Chapter 9 overall. At this point, Soundwave's relationship with his cassettes is strained and they only interact in order to serve Megatron. Only Laserbeak stays loyal to him. Fortunately, it goes better.