It was easier than most would think to disappear on a crowded street. If he really wanted to lose Sherlock he could do so rather easily. Moran allowed himself to be chased, however. He kept to alleyways and avoided the urge to climb up over anything too tall until the sound of sirens were behind them. He turns a corner quickly then and debates disappearing from sight and cornering the detective. He could end the other man quickly here in this empty alley.
He doesn't. It's not his place to decide when or where Holmes dies. That's something reserved for Jim and Jim alone. Sebastian isn't foolish enough to believe that the years the two of them have known each other will be enough to spare him the other's wrath. He doesn't actually want to kill the other man either. So he stands and he waits, gun holstered but never too far out of reach just in case.
"Chasing an armed man through empty alleyways... Normally, I'd say it's not the brightest idea but I'm sure you knew exactly what you were doing." He says. "I won't kill you. I can't." He pauses and gives the other man a teasing smile. "And not just because it'd be a pity to rid the world of such a great kisser."