As far as i understand Legion, the only difference between heretics and hmm orthodox is evaluation of alien technology and the attitude towards leaving others be or not.
There are other differences, although Legion is reluctant to admit them.
Apart from that i always understood that both parts of the geth are identical and share the same networks and datapools. Therefore, if Legion talks about pity, remorse or fear, he talks about all geth.
All geth know fear. We know that because desire for self-preservation is the same as fear of death. Legion only refers to individual platforms that don't need to fear because their programs just get reuploaded into the mainframe. The same way, they don't pity you because the platforms were specifically activated to kill you - the decision to eliminate was made before the activation. Likewise for the remorse - what's been done has been done. The geth don't wallow in past mistakes but learn from them.
And i think it's not far fetched to assume, that if the geth lack these, that they lack empathy etc as well. It would be strange to think that they have some feelings but exactly the ones mentioned above are missing.
It's no question that they have emotions. Observe Legion's reactions and speech properly. There are many emotional ques in there, although Legion often finds itself at loss at defining them. The very fact that they are capable of crafting an ethical guideline that concerns other species demands the ability to emphatize with them.
But what if resources are getting rare? What if organics want to contact the geth? Is it considered an intrusion? Where are the limits? Why not, for example, kill everyone that comes too close to the geth - home? Even just to say "hi"? That would not determine the future of the visitors race but also cause no ethical trouble for the geth either.
Resources are no issue at all. There are more resources in the galaxy than all the organic races together could utilize. Most of the galaxy is still entirely unexplored - as I recall, the Codex mentions that less than 2% of the starsystems have been even vaguely mapped. The geth don't need to hoard garden worlds with inhabitable atmosphere and plantlife - they can prosper anywhere.
And so far they have indeed killed anybody who tries to make direct contact with them. They want to be left alone. This is something you have to get involved with if you want effective allies against the Reapers.
That's a bit shaky ground, though, isn't it? "I'm gonna shoot at you pre-emptively, because someone who looks like you has attacked me before."
Not at all to the geth. They have no friends in the galaxy, nor desire to make any. They see a military vehicle approaching them; the only reason why a military vehicle would be on the planet is to eliminate them - and they're right. That's exactly why you're there - to kill them all.
Again, if this was the Peace-Geth faction, they weren't really ACTING very peaceful. If all you want to do is be left alone, it's not exactly smart to start building listening posts in the Armstrong Cluster (ie, beyond the Veil), start shooting at the first thing that shows up, and then fly in more troops in an effort to wipe out what you're shooting at. None of that yells "We're peaceful!" to me.
Again, this is an important listening post - something that the geth are using to determine whether the organics are planning assaults against
them. An assault on a remote listening post can easily be construed as intent of harming the geth as a whole. Remember, they don't understand or care about organic diplomacy at this point. They have no need for those, because the events that force them to take positive interest to the organics haven't yet taken place. They just want information concerning themselves.
(Of course, we're veering into collissions between gameplay mechanisms and lore here. The Geth were the Enemy in ME1, ergo, they shoot at you. I'm also fairly convinced that the Heretic faction, IF real, did not exist back when ME1 was created. It reeks of something that was added for ME2.)
You do realize that the Mass Effect storyline was written in its entirety before Mass Effect 1 came on the shelves? Only details have been added and omitted since. The big lines have always been there. And really, from narrative point of view it would make no sense whatsoever to just write off Legion's entire character development in favour of some more evil robots to be killed. The Heretics are an important part of the storyline, and plotwise it would be shooting yourself in the foot to throw them out of the window.
I would argue that the Armstrong Cluster was a forward base for the Citadel assault.
Which would make the entire encounter entirely moot, since you were actually seeking for geth that wouldn't be under Saren's control.