# within each group repeat each point
# then slice the first and last out and
# add a variable called linegroup, which provides grouping for start and endpoints of each line
troops %<>% group_by(group) %>%
  slice(rep(1:n(), each = 2)) %>%
  slice(-c(1, n())) %>%
  mutate(linegroup = lapply(1:(n()/2), function(x) rep(x, 2)) %>% unlist) %>%
  ungroup

# create linestring sf object by summarizing the points,
# grab the last survivor and direction value of each group (i.e. the 'endpoint' value)
troops_line <- st_as_sf(troops, coords = c("long", "lat"), crs = 4326) %>%
  group_by(group, linegroup) %>%
  summarise(survivors = last(survivors), direction = last(direction), do_union = FALSE) %>%
  st_cast("LINESTRING")

gp <- ggplot(troops_line) +
  geom_sf(aes(color = direction, size = survivors), show.legend = "line")