While people may not know, Zero was ready to let this man live if only he gave the right answer. He was still naïve enough space by holding back the constructs and reducing the load on Arnold, just for him to speak and answer his questions. Deep down he prayed that Arnold would give him the answers he wanted to hear, even though they are lies. The thought of losing a father figure was not something he was completely prepared for and luckily no one noticed it.
Arnold on his part could tell Zero's true feelings within the bad guy mask. As a man who truly loved Zero as a son, he found this refreshing, and for that, he chose to be honest with Zero with his words. In the end, he couldn't lie to his son and he couldn't even dream of dealing with him.
Bang!
The three constructs finally shattered Arnold's barrier and collapsed on him like an ancient mountain…. Arnold was reduced to mere meat paste as all three constructs slammed into him, dying instantly.