I wouldn't go so far as splicing it in to the ESC Rx leads. Yes, unplug the RED connector from the Rx plug and insulate it. You don't need to cut the wire, just pull it out of the black connector. The black wire on the ESC RX lead is needed to ground the signal wire. It may make sense to splice the BEC power leads into the main ESC power leads to simplify the wiring a bit. But just use the Rx plug on the BEC and plug it into a spare slot in the Rx.
BTW, this is a handy website-
http://scriptasylum.com/rc_speed/_wiring.html.
