Even the murderer can be forgiven -- but first he must repent and ask for that forgiveness through the sacrament of confession. But remember that for that confession to be valid there are several requirements that must be met:
(a) All mortal sins that we remember must be confessed,
(b) There must be true contrition (sorrow) for our sins, and
(c) There must be a firm purpose of amendment (a resolve not to commit the sins again).
One tremendous problem today is that so many people refuse to accept that certain acts are grave sins. If we don't believe something is a sins, how likely is it we will confess it, much less have contrition for it?
Further, how many people treat confession like a car wash: actually planning to sin intending to go to confession before Mass so they can receive Holy Communion. Contrition? Firm purpose of amendment? What is present is a presumption of mercy -- in and of itself a very grave sin.
As for a short time in Purgatory: recall the Blessed Mother revealing to one of the children at Fatima that one of their young friends would be in Purgatory until the end of the world? If a child would incur this much temporal punishment, especially in 1917, can we reasonably expect most adults will incur far less?