Captain Love knew Alejandro was hiding something about himself, but was unsure if he was Alejandro Murrieta or not.
When Diego revealed to Rafael that Alejandro was Zorro, Love was out of the room fetching Elena and we never saw Rafael pass on the information.
When Zorro gave him the "M" cut, it was a confirmation to Love that he was Murrieta. In the heat of battle I'm sure Love didn't take the time to think about Don Alejandro and his suspicions.
Therefore, it didn't occur to Captain Love until he unmasked himself that Zorro, Don Alejandro, and Murrieta were in fact all the same person.
At the expense of overriding the other fairly well thought out replies - try this: stab yourself in the upper abdomen/lower chest area where Love was pierced and then look in a mirror. If your eyes aren't bulging somewhat, stab harder.
Or, if you are in a less lethal frame of mind, try leaning up against a sharp outjutting - like a corner of a cabinet or even a long door knob - against your xyphoid process (the little bony piece at the lower end of the sternum) then, while bearing your full weight on this sharp object - look at your eyes or have someone take your picture and see if your eyes aren't bulging, if even just a tad. I always thought it was to sell the acting job - not a sudden realization of something. Probably my bad, tho......
Whatever doesn't kill me only makes me stronger. How strong are you?
Agreed with the above poster. You've just been impaled by your own sword when you had a guy cornered and disarmed. Wouldn't you be a little shocked? Might have something to do with not being able to breathe properly or that you are realising that you're dying.