Matt Murdock pleaded guilty on Daredevil’s trial for accidentally killing a robber. Elektra then visits him in jail trying to enlist his help in taking down the Hand, but he rejects her on the basis of not trusting her. Before being sent to jail, Matt and Elektra stole billions from the Stromwyns but he caught her keeping a billion for herself.
Elektra used that money to buy Hells Kitchen, outbidding the Stroymwyns and Tony Stark himself. After a walk through the area, she became convinced that the only way to make Matt trust her again is to keep his old block safe while he’s in jail, and the best way to do that was don the Daredevil mask herself.
From – Daredevil Vol. 6 #25