The juxtaposition of the heavenly with the earthly serves to highlight Richard's evil. He is associated with dark, forbidding imagery throughout the play. Here, by association, he is also so portrayed:
Albeit they were fleshed villains, bloody dogs,
Melting with tenderness and mild compassion,
IV:3:6-7
The fleshy, earthy language contrasts with language of virtue. Throughout the passage, words like 'bloody', 'butchery' and 'massacre' are used to taint Tyrell, but also Richard. One of the themes of Richard III is the corruption of rituals and rites. In this passage, the ritual of coronation is tainted. Rather than being anointed with oil, Richard is daubed with blood.
More religious imagery further on describes the children in biblical language:
The most replenished sweet work of nature
That from the prime creation e'er she framed
IV:3:18-19
The comparison could be drawn here to either Jesus, or pre-Fall Adam. The inference, however, is clear. These children are without guilt, and are sacrificed to Richard's lust for power. Their lack of sin is highlighted by the prayer-book by their bedside (an object that Richard will later taint when facing the Lord Mayor).
One of the themes running through Richard III is the nature of beauty. Richard himself lacks physical beauty, and so is '…determined to prove a villain'. The children, on the other hand, are described like statues, flowers, and being of '…summer beauty'. These children, then, are the antithesis of Richard. They are beautiful both internally and externally, whilst Richard is dark and twisted, both mentally and physically.
One should consider also the nature of colour at this point. On Richard, the colour red has bloody overtones, whilst the children are compared to flowers. For Richard, the colour is somehow permanent; his soul is stained by the blood he has shed. The children, however, will soon lose theirs as the 'four red roses on a stalk' fade with hypoxia. The colour white is also a useful indicator. The children are described as being 'alabaster', suggesting spotless cleanliness. It also predicts their pallor in death. This process of decay, when linked with the imagery of roses suggests seasonal change. The very opening line of the play refers back to this: 'Now is the winter of our discontent'.
Something of the nature of the assassins is also revealed. Dighton's nature is suggested by the sound of his name; he is a creature of death. Forrest too suggests darkness. Rather than being a green sylvan ideal, it suggests hidden dangers. Tyrell sounds very much like tyrant, and as such is very much a creature of his master.
When looking at this passage with 21st century eyes it is easy to put a far more sinister slant on this. Coming from a society that invented the term 'compassion fatigue', and in which hyperbolic displays of grief and sadness are becoming the norm (one need think back to the deaths of Princesses Diana and Anne, and even Ken Bigly), one could view this passage with cynicism. This would lead on to another Shakespearian theme; the power and mutability of words.
Imagine a grinning Tyrell, sharply dressed like a lawyer or a politician, delivering this eulogy with an exaggerated rhetorical tone, or perhaps an emotionless monotone, as if he were reading a shopping list. If portrayed as such, it would add a terrifying dimension to Richard III. Richard's vileness is revealed by reference to his social setting. If this baseline were removed, then the outlook would indeed be grim. Alternatively, that he can associate with such inhuman monsters suggests something about his own character.
The passage illustrates a number of themes that run throughout the play, such as the heavenly-earthly dualism, as well as the importance of appearance. They also serve to portray Richard in an unpleasant light. Richard, rather than shunning these themes, seems to revel in them. What could be more earthly, bloody, odious and vile than a growling boar?