Evidence of the relationship is admissible in domestic abuse cases even where such incidentally shows bad character

R. v. Carroll, 2014 ONCA 2:

[122] First, in cases of domestic homicide, evidence may be admitted during the case for the Crown, including evidence of extrinsic misconduct, that elucidates the nature of the relationship between the spouses.  This evidence may tend to establish animus or motive on the part of one spouse, and thus be relevant to prove that the killer of the deceased was the spouse with the animus or motive, rather than someone else, and that the killing was murder:  Moo, at para. 98; R. v. Cudjoe, 2009 ONCA 543, 251 O.A.C. 163, at para. 64; and R. v. Van Osselaer, 2002 BCCA 464, 5 B.C.L.R. (4th) 73, at para. 23.

[123] Second, where extrinsic misconduct evidence is admitted to demonstrate animus or motive towards the very person killed, the usual caution associated with such evidence is not required:  Moo, at para. 100; Pasqualino, at para. 65; and R. v. Merz (1999), 46 O.R. (3d) 161 (C.A.), at para. 59.