Is it possible that the Engineers were able to monitor Earth in between their vi

One of the more logical theories I read was about the fact that maybe the Engineers didn't necessarily want to kill us. That the star maps were left for us to find, and use to one day navigate to the "facility" planet, letting the Engineers know that Earth was ready (?) for the goo. The point driven home by pointing out that when they talk to the Engineer and he becomes enraged, he doesn't worry about killing any the crew, his main focus is to get the goo to Earth as fast as possible. To me, this implies that they were on their way to Earth before Prometheus came. If they were already on their way to Earth, how did they know we were necessarily ready for the goo? You could suggest that they determined this during their last visit but the time in between seems to great for that to be a possibility. By the same logic, you could also consider that maybe they did in fact want to kill us. Maybe they wanted Earth as their own planet. My other thought is more religion based I suppose and that perhaps Earth angered the Engineers with the "birth of Christ". We no longer worshipped the beings that truly created us, and they didn't like this. "God" (Engineers) gives life, "God" can take it away.