Slovik's case was reviewed several times, firstly by an Assistant Staff Judge Advocate to guide Eisenhower's final decision on clemency for Slovik:

There can be no doubt he deliberately decided that confinement was preferable to the risks of combat, and that he deliberately sought the comparative comfort of the guardhouse. To him and those soldiers who might follow his example, if he achieves his end, confinement is neither deterrent or punishment. He has directly challenged the authority of the government, and future discipline depends upon a resolute reply to this challenge. If the death penalty is ever to be applied to desertion it should be imposed in this case, not as a punitive measure or retribution, but to maintain that discipline upon which an army can succeed against the enemy. There was no recommendation for clemency in this case and none is here recommended.

And finally by the head of the Paris JAG (Judge Advocate General):

This is the first death sentence which has reached me for examination. It is probably the first of the kind in the American army for over eighty years - there were none in WWI. In this case the extreme penalty of death appears warranted. This soldier had performed no front line duty. He did not intend to. He deserted from his group of fifteen when about to join the infantry company to which he had been assigned. His subsequent conduct shows a deliberate plan to secure trial and incarceration in a safe place. The sentence adjudged was more severe than he had anticipated, but the imposition of a less severe sentence would only have accomplished the accused's purpose of securing his incarceration and consequent freedom from the dangers which so many of our armed forces are required to face daily. His unfavorable civilian record indicates that he is not a worthy subject of clemency.

By all appearances, Slovik had a fairly brazen plan to avoid combat by getting himself incarcerated for the remaining duration of the war. The army decided that this scheme, if successful, would set an unwelcome precedent for other soldiers to follow if they wished to escape dangerous duties.

JAG lawyers reviewing the case also took account of Slovik's civilian criminal record (though the court martial did not) when determining his suitability for clemency on behalf of Eisenhower (the guy was pretty busy...), indicating that Slovik's past misdeeds (which included multiple custodial sentences and parole violations):

indicate a persistent refusal to conform to the rules of society in civilian life, an imperviousness to penal correction, and a total lack of appreciation for clemency

So, at least in part, Slovik's past caught up with him - the army had actually classed him as unsuitable for the draft at the outbreak of war, due to his incorrigible criminality (and obviously no small amount of stupidity, if I may be frank), but he was reclassified late in the war and called up - probably as a result of a relaxation of standards due to manpower shortages.

Slovik could have dodged the bullet a number of times, and it's apparent that he did not realize the mortal danger he was in until it was effectively too late. His confession was very foolish, primarily because it demonstrated lucidity of thought, premeditation, lack of remorse, and worst of all, contained a threat to repeat the offending - one can admire his earnestness... but, boy was that kid dumb. In a legal sense he really forced their hand, all the way up the chain, authority figures urged him to destroy his provocative confession note, and when it was too late for that, offered to ignore the whole thing if he rejoined his unit. Slovik stuck to his scheme, and the army declined to blink - he made it into a question of: can a soldier simply decline to fight when he pleases? The process was by no means arbitrary, if Slovik had done anything differently (i.e. can the fucking note) his fate most likely would have been similar to many thousands of other deserters/AWOL soldiers.

Sources are primary - JAG records (including text of reviews I quoted) you can find here: https://research.archives.gov/id/6231404?q=*:*

Secondary: Joseph DiMona, Great Court-Martial Cases.

I have an additional question. I am currently in the U.S. Army and have head that it used to be considered an appropriate form of action to conduct a summary execution while in the field(read combat environment). Are there any reputable documentation of this happening? Is it a known about subject?