In many ways the word “energy” is misleading, we are attempting to treat what should be a verb as a noun and this naturally leads to confusion, the word “change” can often be substituted for the word “energy”. Without change there can be no “energy” hence since change requires time “energy” is bound to time.
The various forms of energy are simply various forms of change “potential energy” simply means “potential change”, “kinetic energy” simply means “position change” and so on.
The speed of light represents the upper limit to the speed of change thus it represents a limit on energy.
Conversely we could think of mass as “stability” or “lack of change” the half life of a mass represents the limit to its stability, or perhaps also the degree to which absolute zero is unobtainable.
change and stability are antagonistic, what stays the same can not change and what changes can not stay the same, the mass verses energy equivalence means exactly this.
We can not say what the mass of an exploding car is because if it is exploding it is no longer a car, we can not say how fast a dead engine can go because if its dead we can’t get the required readings. To become a noun a system has to exhibit stability and to become a verb a system has to exhibit change.
We use the word “dance” as a noun to mean a specific type of dance or as a verb to mean the act of dancing but in the end a “thing” and an “event” are different.
all things also have the qualities of an event and all events also have the qualities of things so its a matter of degree rather than absolutes, a block of lead at near absolute zero is very very thing like, it is almost a pure mass, a bit of hydrogen at the temperature of a super nova is very very event like, it is almost pure energy.
As far is is known pure energy can never exist, all events retain mass like qualities, there is always some stability, nor can pure mass exist, all things retain event like qualities, there is always some change
Pure energy would be a dance without dancers just as pure mass would be dancers without a dance, one can not exist without the other.