Why did Maximus not return to his army camp when he escapes being murdered?

Although @John Smith Optional has covered this answer extremely well, I would also add this...

You say:

...why not ride back into the main camp (even if it was mobilizing it's not like a camp of that size could disappear in a day) wrangle up your loyal army, rout the small contingent of soldiers loyal to the emperor, and slay him, then send your best and and fastest riders to catch up to the runners

Since Maximus rode as fast as he could back to his family and was too late, it seems unlikely he could get back to his army camp, somehow convince them to side with him, then send runners from said camp back to his family and somehow still arrive before his family were killed.

The fact Maximus goes straight to his family suggests that he truly understands the immediate peril they are in and realises only he can act.

Maximus had been labelled a traitor to the empire: now led by Commodus, after the death of Marcus Aurelius.

Maximus had been tasked by Marcus Aurelius to reform the Roman state as a republic, handing rule over to the people; this removed Commodus' birthright, hence his slating Maximus for execution.

An Army is loyal to its head of state, in this case Commodus. If Maximus were to return to the command tent of the Germania campaign, he would be doing so to start insurrection and revolution, not simply 'rounding up those loyal to him'.

Even Quintus, his former captain, is utterly powerless to stop his arrest and performs his duty in arresting Maximus (even though he knows he is innocent of the crimes leveled against him).

When common wisdom believes in divine appointment, that Commodus has been appointed ruler by the gods, it would take a huge amount of persuasion to gather any notable sympathy within the camp.

He'd be dead as soon as he showed his face, undoubtedly.

For one thing, the army had likely moved. Roman armies were strategically effective because of their mobility -- they could take down their camp, march 6-8 hours, and set up a new camp each day. This despite their camp including a ditch, earth rampart, wooden stockade, slit trenches, and a standardized layout for all of the legionnaire tents.

For another, the army's loyalty to Maximus is... questionable. Maximus was wealthy, but not "pay an army" wealthy. Commodus had a better claim on legitimacy, as Maximus had not been publicly proclaimed to be the adopted heir of Aurelius. Had Aurelius made that announcement in front of the army, the sheer number of witnesses -- and the fact that adopted heirs were common in Roman society, and Aurelius himself had become emperor based on adoption (as had the previous three emperors, going back 60 years) -- would have made Commodus the fugitive.

And, on the gripping hand, in reality Commodus had been co-emperor with his father for the last two years of his father's life. Commodus already had the titles of "Imperator" and "Augustus".

Because he rushed to save his family first.

He passed out from exhaustion later on and captured by slave traders.

Perhaps if he could save his family, he would return to the camp, or even it was moved he will try to find it.