The Flash’s 150th episode started pretty action-heavy with Barry, Nora, Bart, Jay Garrick, Killer Frost, and even Cisco returning to take down the Godspeed clones, and even then, it only slowed them down momentarily.
That Heart was trapped dormant in the man’s mind, which Barry learned shortly after he arrived.
To that point, Heart seemed like an unassuming and slightly skittish guy, and certainly not someone capable of evil.
It was time for Plan B, which meant Barry (with Cecile’s help) entered the mind of the original August Heart, who had lost his memory.