Small.
Megatron had intended to terminate Starscream that day. He wanted to just take his Fusion Cannon and put it to his head and for it to all be done. However, he just couldn't. Though Starscream had been such a treacherous second-in-command, there was something he couldn't ignore.
Starscream was many things. He was smarmy, cruel, devious, and treacherous. He knew how to use his words in order to get exactly what he wanted, especially since he had that impossibly irresistible gravely voice. He could talk his way out of any situation, which was how he stayed alive so long throughout the war. He could be your best friend, but most of the time he was your worst enemy. He was a treacherous backstabber. However, there was something more in him. It was the reason Megatron had made Starscream his second-in-command in the first place.
Megatron had wanted to get closer to Starscream. When he first met the Seeker, who was part of the royal family, he saw something in him. The seeker didn't have much fight in the beginning. in fact, he appeared as though made of shattered glass, as if one thing could break him. However, the way he carried himself, he always seemed proud, and perhaps just a little seductive. Megatron found this, attractive. He wanted Starscream all for himself, but he knew if he was forward with his true feelings toward Starscream, he would be seen as weak. Therefore, he kept everything to himself.
Of course he did want at some point to tell Starscream how he felt, but through all the beatings that he was giving to Starscream, he could never say his true feelings. He feared that if he did, he would lose all credibility and respect amongst his Decepticon troops. Therefore, he just continued to beat Starscream. That was, until he couldn't.
He tried so hard to terminate Starscream for the final time, so that he wouldn't have to deal with his treachery anymore. However, looking down, hearing his weak little whimpers, he knew one thing. As he looked into the Optics of the one who he had named his second-in-command, only one thing went through his Spark. Slowly, the fusion Cannon lowered, and the Seeker whimpered even more. Perhaps he thought Megatron was going to torture him, make him suffer a slow painful death.
Starscream did not expect the kiss that followed.
Though he was treacherous, conniving, smarmy, and most of all murderous, Megatron could not deny one thing.
He's so... small.
Megatron did not kill Starscream that day, or any day after that. Starscream stayed alive not because of his cunning battle ability or his way with words, he stayed alive because he was so incredibly cute.
