First of all, Rivers, Grey and Vaughan all die together in Act 3, Scene 3. What do they realise as the die? That they're dying because of Richard's whim, and that the curse that Queen Margaret made earlier in the play (which predicted that they'd all meet sticky ends because of Richard) has come true:
Now Margaret's curse is fall'n upon our heads,
For standing by when Richard stabb'd her son.
Then cursed she Hastings, then cursed she Buckingham,
Then cursed she Richard. O, remember, God
To hear her prayers for them, as now for us
And for my sister and her princely sons,
Be satisfied, dear God, with our true blood,
Which, as thou know'st, unjustly must be spilt.
Ominously, Rivers even seems to know there at the end that there's more blood to come - praying for his sister (Elizabeth) and her sons (the two princes in the tower).
Next scene, next death. Hastings, Act 3, Scene 4. And he realises exactly the same things:
O Margaret, Margaret, now thy heavy curse
Is lighted on poor Hastings' wretched head!
O bloody Richard! miserable England!
I prophesy the fearful'st time to thee
That ever wretched age hath look'd upon.
Come, lead me to the block; bear him my head.
They smile at me that shortly shall be dead.
What does this all suggest about justice? That there isn't any actual justice on earth: Richard is just removing everyone who stands in his way. But - and the play bears this out - that Margaret's curse is going to come true of Richard too.