Based on the link provided by @fez in the comments, we can assume a few possibilities:
It's a warning
During the movie we discover at one point that the Engineers decided to release one of the weapons upon Earth. One may think they warned humans in a Biblical sense. Almost like a "Behave or die" meaning. But their plans ran into an end early and we managed to find them thousands of years later.
They never expected humans would go so far
Maybe they planned since the beginning to kill everyone on Earth, way before we were capable of achieving the "Final Frontier", so why bother hiding anything from very primitive life forms?
In this case, they could have expected humans would reach space travel technology sometime, but that wouldn't matter, or they never expected that.
The screenplay doesn't make it clear, but they could have created us (Humans), and reaching to an exact point out in the space based on a cave drawing would be a good challenge to validate our own evolution.
What I want to say, after all of this is that the movie does't answer all the questions it raises, letting us wandering and discussing, so as we can't put ourselves in the place of an extraterrestrial intelligent fictional life form, the best conclusion we will get is: why Humans would or wouldn't do something like that.