He couldn't do it. Why..? Why was it now, of all times, he was hesitating? She was right there, within ten yards. He just had to press a button, just one...simple button. This android was trouble, right? Shutting her down and destroying her meant Cell couldn't achieve his perfect form. They'd win-so why couldn't he just press it already? Was it that mocking peck to the cheek earlier? Maybe her looks? Maybe the fact she wasn't as evil as she was implied to be by Trunks? Maybe she could be helped?

He didn't know what it was, but he couldn't just kill her. She still had humanity left, she had a life of her own before Dr.Gero took that away. Bulma made emergency switches to shut the twin androids down, giving them the perfect opportunity to just finish the androids off, and be done with it. The only issue was, Android 17 was already absorbed, putting Cell in stage two of his development and power. Cell only needed to absorb Android 18 now, and he'd be perfect. So now it was a race. And three ways it could go.

Option 1. He shuts her down, destroys her with one attack, and Cell can't absorb her then, 2. Cell absorbs her, achieves his perfected form, and they're all screwed. Or 3. He spares her, tries to help her escape, she could live as normal a life as she could. And he was stuck on what to do. The right thing, or the thing that would just save earth real quick. It's head vs heart here. He's gotta think rationally about this. But unfortunately, he doesn't have a whole lot of time before Cell finds and absorbs her, since she's like..right there, able to be seen if he turned just so, but thankfully, Vegeta had Cell occupied, the two seeming to be talking. Judging by the look on Trunks face, it was Cell trying to convince Vegeta to let him attain his perfect form before this fight resumed.

So he's gotta make this choice fast, before either Cell spots her, or 18 realizes he's right there and attacks him.