The last line was set to /bin/false. I changed it to /bin/bash but still can't 'su - nagios'. Tried rebooting. The command executes (no error) but a whoami still shows root. Here is how the passwd file looks now.
root

0:0:root:/root:/bin/bash
daemon

1:1:daemon:/usr/sbin:/bin/sh
bin

2:2:bin:/bin:/bin/sh
sys

3:3:sys:/dev:/bin/sh
sync

4:65534:sync:/bin:/bin/sync
games

5:60:games:/usr/games:/bin/sh
man

6:12

/var/cache/man:/bin/sh
lp

7:7:lp:/var/spool/lpd:/bin/sh
mail

8:8:mail:/var/mail:/bin/sh
news

9:9:news:/var/spool/news:/bin/sh
uucp

10:10:uucp:/var/spool/uucp:/bin/sh
proxy

13:13:proxy:/bin:/bin/sh
www-data

33:33:www-data:/var/www:/bin/sh
backup

34:34:backup:/var/backups:/bin/sh
list

38:38:Mailing List Manager:/var/list:/bin/sh
irc

39:39:ircd:/var/run/ircd:/bin/sh
gnats

41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/bin/sh
nobody

65534:65534:nobody:/nonexistent:/bin/sh
libuuid

100:101::/var/lib/libuuid:/bin/sh
syslog

101:103::/home/syslog:/bin/false
landscape

102:105::/var/lib/landscape:/bin/false
bind

103:108::/var/cache/bind:/bin/false
roshan

1000

Roshan Shiwcharran,,,:/home/roshan:/bin/bash
nagios

105:116::/var/lib/nagios:/bin/false
sshd

106:65534::/var/run/sshd:/usr/sbin/nologin
postfix

104:114::/var/spool/postfix:/bin/false
ntop

107:117::/var/lib/ntop:/bin/false
snmp

108:65534::/var/lib/snmp:/bin/bash