What are these objects around François Ravaillac's portrait?

Some of this is relatively easy to understand, some includes speculation.

Edit: LangLangC has found a published description of the engraving, thanks for that.

First, for completeness, a translation of the circumscription and motto:

Franciscus Ravaillart de Angouleme Henrici IIII Francorum regis Parracida sicariorum coryphae
Heu scelus! Heu monstrum tantum qui caedere Regem. Ausus es, an te quis cernere sustineat.

François Ravaillac of Angoulême, patricidal master assassin of Henry IV, king of the Francs
Alas, the crime! Alas, the only monster who would kill the King. How dare you to let yourself be seen.

There is a description of the plate in D. Franken, L'œuvre gravé des van de Passe, Paris (Bapilly) 1881, p. 152. It is not contemporary, so the interpretation is as much bound to its time as mine is.

Ravaillac (François),
Assassin de Henri IV, roi de France. (1578–1610).
A micorps à gauche, en pourpoint uni avec collorette moutante et rabattue; le chapeau sur la tête. Dela main droite il tient un couteau a lame flamboyante. En bord. ov. avec inser. de nom (comme Ravaillart) etc. en latin. Dans les coins des bombes des mêches, des barils de poudrs, et sous l'ov. un chat huant. – Dans la marge infér. 2 l.: Heu...sustineat.

In left half-profile, in plain doublet with turned (?) and folded collar; hat on head. In right hand he holds a knife with flaming blade. In oval border with name inserted (written Ravaillart) etc. in Latin. In the corners fused bombs, barrels of powder, and under the oval a cat hooting. – In the lower margin 2 lines: Heu...sustineat.

I disagree with the fuses. There are some strings dangling down to both sides of the oval, but they are not connected to the flaming objects at the top, and their lower end are pommels. They are drawn completely different from the flames and the smoke in other places of the engraving.

The objects in the top corners are separated from the objects directly to the side of the oval by billowing smoke. These seem to resemble late medieval hand-held canons that are arrested against the ground by means of a tiller mounted to the lower end. The strings connected to them might be pullstrings for releasing a firing mechanism.

In the upper corners there are what Franken describes as "bombs". The left one is clearly a metal contraption with flames and lightning (at the top of the oval) comming out of it, which would probably qualify it as an exploding shell. The right one is just a flaming ball. I would compare it to some sort of fire bomb like greek fire or a fire ball like this one.

The lower left corner shows a shovel (for digging a grave?) and other implements, maybe a scythe (symbol of death itself).

The lower right corner has some wooden stakes and a barrel. Like Franken, I would speculate at it containing gunpowder, and the whole symbolizing the practice of sapping and undermining.

Today, we would associate the term with a metaphorical usage, as in "undermining the authority of a king". Could that have been meant? The engraving was made around 1610. French wiktionary gives a metaphorical usage example from the 18th century (Voltaire). The german Deutsches Wörterbuch der Gebrüder Grimm cites untergraben in the figurative sense from the 16th century, but says for unterminieren that it "appears besides the older untergraben in the 17th century". The engraver is obviously aware of gunpowder. I can imagine him making a point here.

Finally, the animal below the medallion is clearly not a cat. Its large eyes and the beak identify it as an owl, sitting on what could be a candelabra. The owl is a bird of the night, so this association does not say much. In symbolism, there are two opposing interpretations:

• A striga or strix is a bringer of bad omen. Most of the time it is depicted as some sort of winged demon. But another representation has it as an asio, a certain genus of owls.
• The attribute bird of Athena, the goddess of wisdom. While Athena is also the goddess of warcraft, the owl normally does not represent that aspect. Instead it stands for perspicacity, which could be seen as loosely associated with the term coriphae (leader, eminence) which is used in the circumscription to qualify the term sicariae (assassins).
  I am also drawn to this interpretation by a second engraving of Ravaillac. It has exactly the same circumscription and motto, and while the surrounding scenes show the execution, at the top there is also the owl, but this time below a heraldic pavilion, which is normally reserved for sovereignty.

Here is a clipped part with that detail: