Significance of the quote on Deadshot's neck band

The full quote is "I am the Light - The Way". If you look at larger variation of the promo image you used, you can see that the same quote is etched into the barrel of his gun:

Most likely, this is a mis-quote of a famous quote from the Bible, specifically John 14:6:

Jesus said to him, “I am the way, and the truth, and the life. No one comes to the Father except through me.

(It's not uncommon for people quoting from memory to mis-quote "life" for "light", as that is another term commonly associated with Jesus in the Bible.)

In fiction, professional hitmen often claim to view themselves as a "doorway to the afterlife." The intent of the quote, in this context, is that Deadshot is the way that his victims will eventually get to Heaven: by killing them, he is sending them to "meet God".

This interpretation has been confirmed by David Ayer, the director, an an interview with USA Today (thanks to @Leatherwing for the find)

Ayer thinks of it as the villain’s misremembering of a Bible quote from the Book of John, a holdover of his Southern Baptist upbringing. “It all speaks to the layers and history of the character,” the director says. “You can’t get out of the bed in the morning if you think, ‘I’m this really bad, screwed-up person who kills people’ vs. ‘I’m a professional, I’m doing a job and I just help people to the other side.’ ”