Moving without keyframe?

Select the middle keyframe, right-click, open Keyframe Interpolation:

Change Spatial Interpolation to Linear: 

Auto Bezier spatial interpolation almost always produces a more natural movement but it can cause overshoot problems. The trickiest part of solving the problem is figuring which keyframe to change. It is usually the one before the overshoot.

You can have even more control if you learn how to use the graph editor.

I suggest you read the online help and come to grips with AE's concept of separating temporal and value interpolations. Your items move backwards because making a path linear does not automatically make the underlying time linear and vice versa. It can all be fixed easily in the graph editor or for simple situations by selecting the keyframes and changing the type in the right-click menu, but otherwise this is one of those things that you cannot expect to understand right away. Keyframing can take years and years of practice before you're able to actually do cool stuff, which is pretty much what any AE artist or for that matter 3D animators will tell you. As Jose already suggested, try the quick fix and then delve into the more detailed and complicated stuff.

Mylenium

I think the easiest way to revert that is to convert the last Keyframe into a linear keyframe and then into a bezier keyframe. The other way is to manipulate the motion path into the composition panel or into the graph editor panel.