The problem with rattlers is that they don't really "see," they sense heat (hence the general categorization as pit vipers). Small heat signatures are usually interpreted as food. Large heat signatures, like those created by a 200 pound warm-blooded hunter, and generally interpreted as an enemy - the snake knows it can't eat a 200 pound meal. In general, they will try to escape from a large heat signature for two reasons: 1) they know they can't eat it and it may hurt them, and 2) if they have to defend themselves against it they'll use up most or all of their venom and won't have enough to gather a meal for several days. This is why most rattlers, when given enough space, will simply leave. When leaving's not an easy option (like when they're cold or caught off guard, they warn you away, hoping you'll leave them alone.
Of course, there are exceptions to this generalization. Some rattlers don't give you a warning and simply lay still hoping you'll pass. These, in my mind, are the more dangerous ones since you don't know they're there until you step on one and he bites you. There's even some speculation now that humans, in our senseless killing of buzzing rattlers (you know, the ones that are easy to find), are actually artificially selecting for those snakes genetically pre-dispositioned to stay quiet. Maybe killing every noise snake is creating a quieter (on average) species. There is similar speculation related to public land bull elk and reduced bugling.
Of course, there are exceptions to this generalization. Some rattlers don't give you a warning and simply lay still hoping you'll pass. These, in my mind, are the more dangerous ones since you don't know they're there until you step on one and he bites you. There's even some speculation now that humans, in our senseless killing of buzzing rattlers (you know, the ones that are easy to find), are actually artificially selecting for those snakes genetically pre-dispositioned to stay quiet. Maybe killing every noise snake is creating a quieter (on average) species. There is similar speculation related to public land bull elk and reduced bugling.