Agency vs Judged On Actions

I have been questioning a lot of what I believe for awhile. I feel like I don't believe a lot of what the church tells me anymore. I'll probably write a sequence of posts about that and tag them the same.

I can't believe much that we are judged by our own actions and also agency. They seem like they are competing ideas.

Consider a friend I have. He went on a mission and did the whole Mormon thing. He was a very good boy. Then, he decided that he did not want to be LDS anymore. He stopped going to church and even started fighting against the church, bearing testimony to members, "I know the church is false." It seems to me that if his heart had just decided to up and stop working on his mission, he would have been saved in the highest glory of the celestial kingdom. Now, however, he won't. 

Worse, this could've been decided by some external party. Let's say that someone decided to murder my friend on his mission. Now, this person (a murderer) decided my friend's fate by not letting him continue to live. If the murderer kills him now, my friend will not be able to perform actions that he otherwise might have performed to bring him back into the fold of the church.

I hate that the actions of another can decide someone's ultimate fate. It seems so unfair to me. 

It feels to me like life is one big Markovian Process. There are two states--"If I died now, I'd go to the Celestial Kingdom", and "If I died now, I would NOT go to the Celestial Kingdom". Each day, there is some probability of switching between these two states. Our ultimate fate is the state that we are in when we reach a ripe old age and die. During life, we switch between these two states and indeterminate number of times. Given that we switch at least once, a murderer could have decided our fate by killing us before we switched.

We even teach in the church that we can affect others' eternal fate--hence the reason for missionary work. We teach that missionary work is saving souls. Why are those people that missionaries teach judged based on whether or not the missionary chose to serve, or even where that missionary decided to tract that one day (thus knocking on the right door or not), and not their ultimate actions?

The only way that I've been able to reconcile this is that we are not actually judged based on our actions. We are actually judged based on our disposition in the pre-earth life. That means that God already has a judgement for each of us that no manner of actions can change. If our actions do have power, then we run into this phenomenon where a person's ultimate salvation could depend on a murderer deciding the point at which they die.