Mryntu

I've been reading Michael Innes books again (in this case "From London Far"), and I came across the word gratricidal, in the a passage about a Scottish castle...

Castle Moila was famous alike in legend, history, and fiction. To
these courts Magnus Barelegs has brought fire; Donald, Lord of the
Isles, a traitor's promise; Macleod of Lewis a gratricidal knife.
Here had come Prince Charles Edward, thwarted of a throne, and
daughters of a hereditary captain had offered him manchets and wine.

So what is gratricide? Attempts to find this online are stymied by the spelling being corrected to fratricide.

Magnus Barelegs, named in the question, was Magnus Olafsen who ruled in the 11th century. Therefore the 'MacLeod of Lewis' mentioned must be contemporary, or perhaps somewhat later.

The Lewis branch of the clan MacLeod, descended from Leod, a son of Olaf the Black, was a notoriously bloodthirsty family, particularly among themselves :

The traditional progenitor of the Macleods was Leod, whom tradition made a son of Olaf the Black, King of Mann and the Isles. Tradition gave Leod two sons, Tormod - progenitor of the Macleods of Harris and Dunvegan (Sìol Tormoid); and Torquil - progenitor of the Macleods of Lewis (Sìol Torcaill). In the 16th and early seventeenth centuries the chiefly line of the Clan Macleod of The Lewes was extinguished due to family infighting. This feuding directly led to the fall of the clan, and loss of its lands to the Clan Mackenzie. The modern line of chiefs of Clan Macleod of The Lewes are represented by the leading family of a cadet branch of the clan - the Macleods of Raasay.

ScotWeb

Thus the reference is almost certainly a misprint for 'fratricide'.

Note : As one of the Johnstones of Annan (Numquam non Paratus - 'never unprepared') whose ancestors were prone to sneak over the border and raid the English, one can make no criticism of other Scottish clans.

It's a typo:

By his MacKenzie wife, Allan [MacLeod] had three sons, one of whom, Alexander, succeeded him. His second wife was a daughter of Roderick MacLeod of Lewis and bore him a son, Roderick, whose name, to quote Morrison (1968-76. section V) is synonymous with 'soaring ambition, crooked counsels and bloodthirsty deeds'. Allan's two brothers who lived in Lewis resented the MacKenzie alliance, came to Gairloch and murdered Allan and the two younger boys.

"THE MACLEODS OF LEWIS AND OF ASSYNT, COIGACH AND GAIRLOCH"

Here, gratricidal is a typographical error (or in short "typo") of the word "fratricidal."

Fratricidal according to Merriam-Webster is

one that murders or kills his or her own brother or sister or an individual (such as a countryman) having a relationship like that of a brother or sister.

Following website₁ present all the typographical errors associated with the word Fratricidal.

dratricide, rratricide, gratricide, vratricide, cratricide, featricide, fdatricide, ffatricide, ftatricide, frqtricide, frwtricide, frstricide, frztricide, frarricide, frafricide, fragricide, frayricide, frateicide, fratdicide, fratficide, fratticide, fratrucide, fratrjcide, fratrkcide, fratrocide, fratrixide, fratridide, fratrifide, fratrivide, fratricude, fratricjde, fratrickde, fratricode, fratricise, fratriciee, fratricife, fratricice, fratricixe, fratricise, fratricidw, fratricids, fratricidd, fratricidr

So, fratricidal knife is a knife which is used to murder/kill their own siblings or to murder/kill individuals having a relationship like that of a brothers/sisters.

_{1: scroll down the bottom of the page.}