Penal Code Chapter 224
Each of several
persons liable for an act done by all, in like manner as if done by
him alone
34. When a criminal act is done by several
persons, in furtherance of the 
common
intention of all, each of such persons is liable for
that act in the same manner as if the act were done by him
alone.
http://statutes.agc.gov.sg/non_version/cgi-bin/DisplayContent.pl?DOCID=2008-REVED-224&VID=1228207122-000355&WEF=latest&TYPE=simple&mode=and&version=currentVersion&query1=common%20intention
For a robbery, there is usually a getaway driver. If all get
caught, the getaway driver will get same punishment as the people
robbing the bank, even though he is not physically in the bank
during the robbery. Same for the mastermind who is not even
there.
So it is possible all will get charge for murder for the
scenario you said.
I disagree.
For the first robbery example, you are right in saying all will
be charged for robbery, but not necessarily same punishment.
Depends on the role played. (Reason why in a murder case, all
will be hanged is because of mandatory death sentence for
murder)
For the scenario provided of a group of ppl including the
shopkeeper beating the shoplifter to death, they will never
be charged with murder. Prima facie, it is a murder case.
But, there are defences available to all of
those involved in the killing.
s300 Exception 2.—Culpable homicide is
not murder if the offender, in the exercise in
good faith of the right of private defence of person or
property, exceeds the power given to him by law, and causes
the death of the person against whom he is exercising such
right of defence, without premeditation and without
any intention of doing more harm than is necessary for the
purpose of such defence.
They will not be charged with murder unless one or more in the
group is using the opportunity to exact revenge. Even then, only
that particular person or few persons wil lbe charged with murder
cos the yshare a common intention to kill. It is unlikely that in
the absence of evidence to show that those involved was exacting
revenge for personal grudge, the courts will impute an intention to
kill as their common intention.
And definitely the shopkeeper and the member of the public does
have a right to private defence.
Right of private defence of the body and of
property
97. Every person has a right, subject to the
restrictions contained in section 99, to defend —
(a) his own body, and the body of any other person,
against any offence affecting the human body;
(b) the property, whether movable or
immovable, of himself or of any other person,
against any act which is an offence falling under the
definition of theft, robbery, mischief or criminal trespass,
or which is an attempt to commit theft, robbery, mischief or
criminal trespass.
Acts against which there is no right of private
defence
99.
(3) There is no right of private defence in cases in
which there is time to have recourse to the protection of the
public authorities.
Do note that if the Police is around, the shopkeeper is not
allowed to beat the shoplifter.
Definitely the shopkeeper is legally entitled to detain a
shoplifter in the course of defending his property. As to the use
of force to restrain the shoplifter, it all depends on
whether the use of force was reasonable and whether more force was
used than necessary. If reasonable force, not an offence at
all.
Nothing done in private defence is an
offence
96. Nothing is an offence which is done in the
exercise of the right of private defence
But if shopkeper killed the shoplifter, he will be deemed
as exceeding the right of private defence, charge may
be culpable homicide not amounting to murder but not
murder. But sometimes due to policy reasons and in the absence
of protests from the shoplifter's family, the AG
chambers may not even prosecute the shoplifter. (This is especially
so if the shoplifter persistently resist attempts to restrain him
and even hurt the shopkeeper in the process)
