The Greek word for robber in John 18:40 is λῃστής. This word is defined by Strong's Enhanced Lexicon this way:
3027 ἀρχιλῃστής, λῃστής [lestes /lace·tace/] n m. From leizomai (to
plunder);15 occurrences; AV translates as “thief” 11 times, and
“robber” four times. 1 a robber, plunderer, freebooter, brigand.
The Greek word used as murder here is φόνος. It is defined as:
5408 φόνος [phonos /fon·os/] n m. From an obsolete primary pheno (to
murder); GK 5840; 10 occurrences; AV translates as “murder” eight
times, “slaughter” once, and “be slain + 599” once. 1 murder,
I don't think these words shed much light on the core of your question. These passages don't conflict though, I think the clearest explanation is that Barrabas was both a thief and a murderer and that none of the authors intended their description of his crimes to be exhaustive.
Strong, J. (1996). The exhaustive concordance of the Bible : Showing every word of the text of the common English version of the canonical books, and every occurrence of each word in regular order. (electronic ed.). Ontario: Woodside Bible Fellowship.
To build on blundin's answer, the most likely sense of the word in this context is "brigand". The NET Bible includes this footnote:
It is possible that Barabbas was merely a robber or highwayman, but more likely, given the use of the term ληστής (lhsth") in Josephus and other early sources, that he was a guerrilla warrior or revolutionary leader. See both R. E. Brown (John [AB], 2:857) and K. H. Rengstorf (TDNT 4:258) for more information. The word λῃστής was used a number of times by Josephus (J. W. 2.13.2-3 [2.253-254]) to describe the revolutionaries or guerrilla fighters who, from mixed motives of nationalism and greed, kept the rural districts of Judea in constant turmoil.
There are several ironies here:
Barabbas was almost certainly guilty of attempting to overthrow Roman rule and had no doubt received whatever due process was common at the time. Jesus, the gospels (especially John) make clear, was not interested in political, but spiritual revolution. His followers were at pains, after his crucifixion, to distance themselves from the sort of rebellion that eventually resulted in the destruction of Jerusalem. The trial of Jesus (especially in John) comes of as a trio of kangaroo courts where Jesus couldn't catch a break and didn't even seem to try to defend himself.
The name Barabbas in Aramaic means “son of abba,” that is, “son of the father,” and presumably the man in question had another name (it may also have been Jesus, according to the textual variant in Matt 27:16, although this is uncertain). For the author this name held ironic significance: The crowd was asking for the release of a man called Barabbas, “son of the father,” while Jesus, who was truly the Son of the Father, was condemned to die instead.
The high priests are so instant the Jesus be crucified that they declare, “We have no king except Caesar!” It's the sort of thing a person would heartily regret saying the next morning on further reflection.
I should also point out that we tend to divide crimes more finely than was common in the past. We have several levels of illegal killing and many different designations of illegal taking of another's property. Punishments are usually tied to the severity of the crime. But in the past, there was less emphasis on differentiating the circumstances of various crimes and execution was the typical punishment for a wide range of crimes. In the case of highwaymen and brigands who demanded "Your money or your life", the only distinction between the one crime and the other was how the victim responded (in theory).
The Romans usually only crucified insurrectionists. That is, people who had an agenda to harm their empire. "Thief" or "robber", therefore, is unfortunate language as we interpret that in our culture as someone who steals. But, Barabbas and those crucified alongside Jesus were more likely thought of by the Romans as Terrorists.
What we know historically, contextually and culturally therefore gives us the best clues to interpreting these words/phrases.
THE POLITICS OF JESUS CRUCIFIXION:
I believe that high level politics was played, which let to the crucifixion of Jesus Christ. I read from some ancient records which explained that Yeshua Barabbas (Jesus the son of the father) was not just a robber and killer as portrayed in some gospels, but a rebel trying to overthrow the roman authority and free the Jews. He had a guerrilla warfare group who constantly ambushed, attacked and killed roman soldiers and burn their homes. This must have endeared him to his Jewish folks as their redeemer. The Jews believing Yeshua Nazareth (Jesus of Nazareth) - the Christ (Redeemer of mankind) to be the king that would totally redeem them from the Romans tyranny wanted him crowned king, but Jesus escaped and refused to the crowned king, which led to the disappointment of the Jews in him because they thought he was an earthly king, which Jesus Christ personally denied and stated that his kingdom is not of this earth but spiritual and heavenly.
Pilate must have been aware that the people would prefer Barabbas to be released as against Jesus Christ because he fights in their side to dethrone the Roman authority. He did confide in his wife that "I have quelled rebellions in this ranging out-post a number of times and Caesar had promised me that the next time it would be my blood." Even after Jesus Christ asked him if he knew the truth, he asked his wife if she would recognize the truth when she sees it. His wife told him one wouldn't know the truth unless you agree that it be told to you. He answered his wife that the only truth he knew at that moment was the rebellion that was about to happen - it showed that he was more interested in holding on to his office rather than accept the truth. He said if he released Jesus Christ, there would be rebellion, and even if he condemned him, Jesus Christ' followers could still raise a rebellion. I believe he deliberately made the choice to have Jesus Christ crucified because his supporters were fewer in number compared to those of the High Priests'.
I believe that mere washing of hands did not completely exonerate Pilate from being culpable in the murder of Jesus Christ. Was there a real trial? People shouting "crucify him" isn't a real trial. Pilate just ordered him to be crucified without proper trial.
However, it was an earthly destiny which Jesus Christ came to fulfill.