The gist of the story suggests that the “old prophet” was about to die himself. It’s quite possible God did kill him (or Jeroboam; see later!) soon after he made arrangements to bury the younger prophet in honor – an honor the younger prophet might not have gotten had he died later.
This is one of those stories that stand as evidence that even a legitimate prophet of God can mislead people as a prophet of God, and be in rebellion (whether well-meant or not) against God. The underlying word for “disobey” is “rebelled against” and it’s the same word used of Aaron and even Moses at Meribah in Numbers 20:24 and 27:14.
Presumably, though, the younger prophet, being under fierce temptation not to eat or drink until he returned home, should have discerned that either this guy was lying for some reason or had himself been fooled by a devil masquerading as an angel. Also, the old prophet didn’t say he was passing along a command to the younger prophet; and didn’t even say it was the angel or presence of YHWH himself but only an angel. God might make allowances for previous orders, but He wouldn’t send mere angels to announce that. Moreover, the old prophet was himself living in the same Israel city of Bethel where Jeroboam was worshiping with the altar – but neither he nor his sons had been asked to do anything there, which at best casts aspersion on his honor that some other prophet would be called to do so, and even suggests he was complicit in the idolatry going on. The younger Judean prophet should have been suspicious when this guy from Bethel shows up on the road and invites him back to Bethel to eat and drink. (Which also by the way would void the command not to go back by the same road.) Note that the old prophet makes his lie about the angel after hearing the command of YHWH Himself – the younger prophet should have been suspicious that the older prophet wasn’t at least confused why a mere angel would come to him making an offer against the command, or perhaps seeing it as a test and so (since there wasn’t an anti-command involved) encouraging the starving young prophet to keep going, maybe with support provided by the old prophet.
In fact, the old guy should have been notorious already to the younger prophet, if 2 Chronicles 11:16-17 correctly reports the situation. Jeroboam had been interfering with priestly families caravaning down to Jerusalem to serve their courses in the Temple there (and otherwise interfering with any local priestly duties of theirs, appointing other people as idolatrous priests instead), so all the priestly families had packed up and moved down to Judah, inspiring anyone who devotedly cared about God (possibly including this young prophet who knows his way to, from, and around whichever Bethel this is in Israel) to migrate south, too. (At the time Reheboam was acting much more faithfully to God.)
So who is this old prophet still in Bethel with his sons and/or disciples? He’s someone who should have been opposing the situation but who hadn’t been, or Jeroboam would have been persecuting his school. This is something that should have been at least suspiciously suspicious to the younger prophet. It turns out later that this guy already had a tomb reserved near the Bethel idolatrous altar along with the priests serving there!!
In a way things turn out as well as they could under the circumstances, since the old prophet actually comes to penitently honor and validate the younger one, and takes a public stand in validating him. Jeroboam had already tried to kill the younger prophet for speaking out against the idolatry. The older guy there in the same place, just volunteered to be killed by the evil king.
Later (in 2 Kings 23) when Josiah arrives to fulfill the prophecy, he burns the bones of the priests and prophets buried near the idol’s altar of Bethel (actually burns them on the idolatrous altar) – but spares the bones of the younger and the older prophet, for both of them had stood up against the altar and had prophesied his coming.