ok the green monsters in half life 2 the ones that help u and the ones that mounted that machine gun on ur boat the ones that shoot green volts of lightning. Ok now what u know what i am talking about, i noticed that in half life 1 they are bad and in half life 2 they are good? can someone explain?!

In HL1 the voritgaunts are being controlled by the Nihilanth (note the shackles on them in HL1), the Nihilanth in turn is being controlled by the Combine. Now that the Nihilanth is dead the vortigaunts are no longer being controlled. Now during the portal storms and the 7 hour war they got teleported to earth, along with headcrabs and other Zen wildlife, where they then allied with the humans against the Combine.

I just wonder what happened to the aliens from Half life Opposing Force ...and that poor trooper that's still being held by the G-man.