a common unlawful purpose. Or in the language of the section
forming “an intention in common to carry out an unlawful purpose and to assist each other therein”: R. v. Simpson, 
1 S.C.R. 3,  S.C.J. No. 4, at p. 15 S.C.R.; Simon, at para. 40.
 Section 21(2) extends liability to persons whose participation in the offence actually committed would not be captured by
s. 21(1). But these persons must have participated in the original
common unlawful purpose with others and either knew or, in
most cases, should have known that one or more of the other participants in the original purpose would likely commit the offence
in pursuing their original purpose: Simon, at para. 41.
 The offence to which s. 21(2) extends liability is not the
“unlawful purpose” to which the subsection refers. The offence
is a different crime. It is a crime that a participant in the original “unlawful purpose” actually commits in carrying out that
purpose. In other words, it is a crime incidental or collateral to
that original unlawful purpose. The purpose must be different
from the offence committed: Simpson, at p. 15 S.C.R.; Simon,
at para. 42.
 Section 21(2) also contains a knowledge or foresight
component expressed in the alternative: “knew or ought to
have known”. In other words, knowledge or foresight may be
established on a subjective or objective basis. Despite these
statutory alternatives, when the offence committed is murder,
the knowledge or foresight element may only be satisfied by
proof that the accused actually foresaw or actually knew that
another participant in the common unlawful purpose would
kill another with either state of mind necessary to make the
killing murder: R. v. Laliberty,  O.J. No. 2808, 117 C.C.C.
(3d) 97 (C.A.), at paras. 35-36; see, also, R. v. Ferrari,  O.J.
No. 2649, 2012 ONCA 399, 287 C.C.C. (3d) 503, at para. 61.
 Unlike s. 21(1)(a), which deals with persons who actually
commit an offence, s. 21(2) and its components:
— agreement or purpose
extend liability to persons for offences committed by others: Simon,
at paras. 40-43.
The principles applied
 Despite the unwarranted intrusion of s. 21(2) into the
instructions on second degree murder and attempted murder
by the reference to its application “to all counts”, as well the