Some hold that it is murder other's do not. This is (some of the list ranging from most lenient to strict)
. I do not mean to say that this or that posek is the only one who holds that particular opinion.
Tosfos - abortion is Muttar
Ran and Rosh - abortion is muttar as long as you are not putting the mother in physical danger by terminating the pregnancy
Ramban, Ramah, Behag - Abortion is a bad thing because we are preventing the existence of a future Human being and shomer Mitzvos, but it is still permitted to do in extenuating circumstances
Maharit - Abortion is assur because one is wounding, or causing a chavallah, in the pregnant mother. According to this approach, abortions would be permitted for constructive purposes, in the same way that one is allowed to wound oneself for a constructive purpose. Some would extend this even further and say that a woman can abort her own fetus, just as she is allowed to wound herself.
Rav ZN Goldberg 's understanding of the Rambam - that abortion would be prohibited due to chavallah of the fetus (this would be an additional reason for the prohibition, above whatever understanding you may have of the Rambam, and above other reasons suggested by HaRav Goldberg in other places. I am not claiming that HaRav Goldberg holds that this is the only reason why an abortion may be prohibited. - see our lengthy discussion of the Rambam's opinion here)
According to the Ohr Sameakh's understanding of the Rosh, one who performs an abortion without the permission of the parents is transgressing the prohibition of gezeilah, stealing.
According to HaRav SZA ZTL one who aborts a fetus is guilty of gezeilah from the fetus him/herself. Whether or not the parents would be allowed to give permission remains an open question.
Chavos Yair, due to the prohibition of wasting seed (which according to him is a torah prohibition)
According to the Mizrachi and Rav Chaim Palagi and their interpretation of the Rishonim, the Chachamim declared that a Jew is prohibited from performing an abortion because a gentile is prohibited, and it would be inappropriate for a Jew to be allowed to do something that is prohibited to a gentile. They explicitly and vehemently reject the possibility that it has anything to do with Shefichat damim (murder).
According to R Yaakov Emden, abortions are prohibited because of waste, which is a rabbinic aspect of the HZL prohibition; therefore they can be performed in circumstances of great need. If the child is illegitimate from a relationship that would incur the death penalty, the RYE would allow it outright.
The opinion of the Beit Yehuda and the Chofetz Chaim (according to his interpretation of Tosfos) that it was a rabbinic decree and the reason for this decree was because it is akin to murder.
Abortion is a rabbinic decree because permitting abortions would encourage promiscuity (Chavos Yair 31)
Abortion is a rabbinic decree because it would be a negation of the mitzvah of P’ru U’rvu (the obligation to procreate) (Mishpetei Uziel here)
Abortion was a rabbinic decree because it will prevent the potential of a future life (Mishpetei Uziel here)
Abortion was a rabbinic decree because by terminating a pregnancy, one prevents a future soul from doing mitzvos (Ohel Moshe by Rav Moshe Zweig Chelek 3 page 49)
Abortions are considered morally wrong by other religions, and thus we should also take a moral stand (Ohel Moshe, see above)
The opinion of Rav Moshe Feinstein that it is indeed prohibited due to murder
The opinion of the Maharam Shick and Sdei Chemed that it is Avizrayhu of retzichah due to chatzi shiur
The opinion of the Minchas Avrohom that it is due to murder but he differs from RMF by only applying this rule if the fetus is viable
The opinion of the Meshekh chochmah and Moshav Zekeinim LeDaas Baalei Tosfos who hold that it is murder and liable "medin shamayim", and the drasha of rabbi yishmael only exempts Jews from punishment from Beit Din
One more opinion is that of the Zohar, which I did not discuss because it is unclear from the Zohar what the Zohar actually holds from a Halachic perspective. The Zohar speaks about how terrible abortion is because it is ruining the creation of God. (see` Zohar Shmot 3:2)
http://rationalistmedicalhalacha.blogspot.com/2011/02/now-that-i-have-completed-my.html ( He then goes on to explain different time periods) I'm just bringing these opinions not saying which way I believe, because I simply do not know.